''American Playhouse'' on PBS is proving to be the most memorable dramatic series on television since the early days of ''Playhouse 90.'' Although not every program on this series, which normally airs on Tuesday nights, has been superb, in every instance viewers could be assured of intelligent, sensitive, stimulating programs. In no case during the past season could an evening with ''American Playhouse'' be considered a waste of time. Such events as ''The Ghost Writer,'' ''Pudd'nhead Wilson,'' ''True West,'' and ''The Cafeteria'' in one season would be quite enough to justify the existence of such a series. But in addition there have been and will be superb pick up programs from other sources, such as ''The Gin Game,'' ''Hughie,'' and ''Heartland.''
Next week and in the weeks to come ''American Playhouse'' will be dealing in provocative, controversial reality translated into drama. Concealed Enemies (PBS , Monday, Tuesday, and Wednesday, May 7 (9-11 p.m.), 8 (9-10 p.m.), and 9 (9-10 p.m.) is based upon the famous Alger Hiss-Whittaker Chambers controversy in which Alger Hiss was eventually found guilty of perjury. Then, May 15 through June 26, ''American Playhouse'' will reprise a seven-part prizewinning drama that explores the life of J. Robert Oppenheimer, the controversial ''father of the atomic bomb.''
''Concealed Enemies'' is an attempt at evenhandedness, although it is admittedly very difficult to maintain such an approach in the Hiss-Chambers affair. The carefully constructed, step-by-step script written by Hugh Whitmore and directed by Jeff Bleckner tries to take into account the framework of history, the communist vs. fascist alignment of most of the world, the political aspirations of some of the chief participants, the psychological profiles of the major figures.
But so complex is the politics, so confusing the psychosexual accusations, so disturbing the confrontations with truth, that even viewers who spent much time in the 1948 period of the trials are bound to emerge from this four-hour immersion as ambivalent as ever.
Hiss, a prominent member of FDR's New Deal entourage and president of the prestigious Carnegie Endowment for International Peace, was accused of being a communist, of passing on classified State Department papers to Whittaker Chambers, an admitted ex-Communist and senior editor of Time magazine.
Hiss not only denied the accusations, he denied knowing Chambers. Although he could not have been convicted of any criminal offense because the statute of limitations had run, Hiss insisted upon suing Chambers for libel and appeared voluntarily before a federal grand jury, an action that resulted in the perjury charges against him. He was found guilty eventually.
Those who want to believe Hiss will find something in the program to strengthen that belief; those who wish to believe Chambers will also find much to strengthen that belief. However, some new evidence previously withheld by the Federal Bureau of Investigation and now dubbed ''forgery by typewriter,'' plus the material bearing on Chambers's alleged homosexuality, tend to strengthen the Hiss case, while there is little to strengthen the already strong Chambers case. That seems to remain intact despite a recognition of psychological disturbance. But it would be difficult to believe that anybody on either side could come out of the experience of the film absolutely certain about one side or the other.
If ''Concealed Enemies'' has a recognizable style, it is, perhaps, Hiss's style - with Edward Herrmann carefully underplaying the seemingly emotionless Hiss. There is an obvious attempt to hold down the melodrama, although certainly the elements are there. Only Peter Riegert as Richard Nixon tends to overplay a bit, sometimes sounding more like Groucho Marx than Nixon.
For those fascinated by Greek tragedy, by the never-ending search for fact, by the mysterious quirks of the human psyche, ''Concealed Enemies'' may be a maddening experience, a jigsaw puzzle with several pieces missing. For those suspicious that the American judicial system can be influenced by current events , ''Concealed Enemies'' may be especially meaningful.