Los Angeles — ''Masterpiece Theatre'' fans may be amused to discover that ''Mouseterpiece Theatre'' is one of the most popular series on the Disney Channel. Now in its fourth month of operation, and already the fifth-largest pay-cable service, the Disney Channel has attracted more than 300,000 subscribers in over 800 cable systems in 49 states. They pay between $7 and $12 monthly for the seven-day-a-week, 7 a.m.-through-11 p.m. service, which is now available to basic cable systems that ''pass'' (a trade term meaning ''is available to, if they wish to pay for it'') more than 7 million homes.
In a recent Monitor interview, Ron Miziker, vice-president in charge of programming, explained to me that the Disney Channel shows many of the old Disney favorites but that around 15 of the universally hailed classics like ''Snow White'' and ''Bambi'' are being kept off the air until the US Supreme Court clarifies the status of home videotaping, since it is apparent that these Disney favorites would be prime home-taping candidates. (The court recently put off making a decision until next year.)
The Disney people want to avoid home-taping of those films, because on commercial cassettes and in theatrical release they remain huge potential moneymakers for Disney.But a few such favorites as ''Mary Poppins'' and ''Alice in Wonderland'' are already part of the Disney Channel repertoire of films.
According to Mr. Miziker, however, surveys have shown the surprising fact that the Disney film requested most often is ''Old Yeller,'' which has become a standard part of the new channel's programming.
Mr. Miziker believes that television is sometimes a disturbing or divisive force in the home. ''One of our goals is to see that our channel will never play that role in households. We are trying to reach the broadest number of people with what we feel are programs that are enjoyable, informative, and always constructive. We will never show on our channel any program which the whole family cannot safely view.
''Of course, there will be some which are better viewed with parent present, and some which are strictly children's shows. But there's nothing a parent ever has to put on a personal banned list.''
Probably the best-known program on the Disney Channel is the ''Mouseterpiece Theatre.'' It's a tongue-in-cheek daily cartoon showcase, hosted by George Plimpton, of vintage Disney cartoon classics, with Plimpton making like Alistair Cooke.
Mickey Mouse plays an important role in much of the Disney programming - every day opens with ''Good Morning, Mickey,'' a one-hour cartoon show featuring Mickey, Donald Duck, Pluto, etc., from the Disney archives. There's also a daily exercise show called, you guessed it, ''Mousercise.''
The cablecast day always ends with a movie, usually from the Disney library. In between are such things as a regular ''EPCOT Magazine'' show based on human-interest stories from EPCOT Center in Orlando, Fla., and other parts of the world; lots of animal shows; a ''Pooh Corner'' show for preschoolers; and variety shows. The new fall schedule will premiere a college talent show, an Australian adventure series, a science and problem-solving program hosted by a pair of 14-year-old twins (a boy and girl), and ''Donald Duck Presents'' (an animated educational show).
What would make the Disney Channel a success by Mr. Miziker's standards?
''Well, our goal is entertainment primarily, although if in the process of entertaining we also inform, that's fine. What I get a lot of gratification from is the response we got back on our first survey of viewers. People who subscribe watch the channel about 211/2 hours per week. And out of that, 81/2 hours are being watched by the head of the household.
''That tells us that the whole family is watching . . . and enjoying. The satisfaction level of the programming was very high, with 73 percent rating the programs excellent or very good.''
Will the Disney Channel ever do anything that will be for adults only?
''At this point, no.''
Would Miziker recommend that cable families subscribe only to the Disney Channel?
''No. In most cases our channel will be one of several subscribed to. We are just offering another choice. I wouldn't want to regulate the country's morality. Maybe some homes want R- or even X-rated material. All we offer is a range of programming which parents can always feel safe with, safe to walk out of the room and leave the kids there to watch and not be concerned.''
Does the Disney Channel carry news programs?
''No news at the moment. But maybe in the future, yes. Also, we don't do public-affairs documentaries, per se. We won't air documentaries on drug abuse and heart attacks and things like that. There are other channels that do things like that.''
Aside from ''Old Yeller,'' what other Disney attractions do people want to see the most?
''They ask for films about the theme parks. And the animated classics. Perhaps we will run some of them as a special event in the course of the year, despite the pending legal action. And the old 'Mickey Mouse Club' is still a great favorite. All the Mickey Mouse cartoons rank high - and we show them often.''
Is there any modern Disney character that comes close to matching the popularity of the old familiar characters?
''No. Although Pooh has been very successful, it is not an original Disney creation. But Mickey Mouse is still the No. 1 cartoon and I guess always will be.''
The Disney Channel Magazine, which is provided to subscribers to the Disney Channel, includes a programming guide as well as special features. ''We try to make the magazine as interactive with the programming as possible. For instance, in the case of one show we are doing, 'Do It Yourself Detective,' you wouldn't know who solved the crime if you didn't have the clues that are in the magazine.'' I looked over some issues of the magazine and it appears to be a colorful, skillfully edited, full-size, slick-paper, four-color publication containing lots of activities for youngsters. The appeal, however, is strictly to children.
How can TV viewers obtain access to the Disney Channel?
''Well, first of all, they must subscribe to their local cable system, assuming there is one. Almost 40 percent of all American households now are wired for cable. Then they can call their cable company and demand that the system make the Disney Channel available. In Nashville, there was actually a petition circulated among cable subscribers when the local cable company didn't offer our channel.''
Does the Disney Channel have a slogan?
Mr. Miziker smiles. ''No, not yet, unless it's the Mouseketeers theme song. But here's something said about us which I like to quote because it is such an apt description: Something old, something new, something borrowed, but nothing blue.''