A comic gem returns to Broadway; You Can't Take It With You Comedy by Moss Hart and George S. Kaufman. Starring Jason Robards, James Coco, Elizabeth Wilson, Colleen Dewhurst. Directed by Ellis Rabb.

One of the American theater's most endearing and most enduring comic creations has come back to Broadway at the Plymouth Theatre with its mirth intact and its merriment undiminished. If one play can restore an era of good feeling, ''You Can't Take It With You'' is that play. The 47-year-old farcical comedy is as robust and hearty as it was when Moss Hart and George S. Kaufman first presented it to a welcoming world and a responsive Pulitzer Prize Committee.

Eccentricity is a way of life in the household headed - but not dominated - by grandpa Martin Vanderhof (Jason Robards). The touchstone of humanity that makes these freewheeling spirits credible and not simply laughable is their decent respect and genuine affection for one another.

In their stage directions, Hart and Kaufman acknowledged that to call the scene of the action a living room ''is something of an understatement. Here meals are eaten, plays are written, snakes collected, ballet steps practiced, xylophones played, printing presses operated. . . . This is a house where you do as you like, and no questions asked.''

Grandpa, who quit business 35 years ago to enjoy life, now has time to attend commencements, visit the zoo, polish his darts game, and read. For logical reasons of his own, he also ignores the Internal Revenue Service. Daughter Penny (Elizabeth Wilson) used to paint but took up playwriting because somebody delivered a typewriter to the house by mistake. Son-in-law Paul and permanent house guest Mr. DePinna (Jack Dodson and Bill McCutcheon) make fireworks in the basement, inadvertently supplying some well-timed explosions.

But the authors have not done with their cloud cuckoo's nest of amateurs. Besides her balletic efforts, fey granddaughter Essie (Carol Androsky) makes candy which husband Ed (Christopher Foster), the resident xylophonist, distributes around the neighborhood along with inflammatory slogans printed on his own press.

The straight member of the family is Alice Sycamore (Maureen Anderman), who not only holds a secretarial job in Wall Street but is being courted by the boss's son, Tony Kirby (Nicholas Surovy). The Vanderhof cottage industries are in full swing when Tony and his parents (Richard Woods and Meg Mundy) arrive - on the wrong evening - and are subsequently carted off to jail when G-men raid the premises. Since ''You Can't Take It With You'' is a fairy tale for moderns, however, a happy ending comes with the territory.

Ellis Rabb's staging is both antic and true. It radiates good feeling. Mr. Robards's Grandpa Vanderhof is a benign presence for whom ''laissez faire'' means harmless self-expression. Miss Anderman's Alice recalls the era when heroines were a little breathless and terribly sincere. She is admirably matched by Mr. Surovy, who declares Tony's love and independence with equal ardor.

Miss Wilson is a delight as the multi-untalented Penny, who wreathes her lack of tact in a seraphic smile. Colleen Dewhurst ornaments the brief role of the Grand Duchess Olga, currently a Childs Restaurant waitress, with a thick Russian accent and an imperial air, with a feathered hat to match. As her fellow countryman in exile, ballet master Boris Kolenkhov, James Coco, exults in Slavic insults and wrestling prowess. Other members of a large and excellent cast include Rosetta Le Noire and Arthur French as the black domestics of the household, Orrin Reiley as a confounded IRS man, and Alice Drummond as a tipsy actress.

James Tilton's scenery and lighting and Nancy Potts's costumes express visually the care and devotion to detail which distinguish this wonderful revival of a modern American classic. At the curtain calls, when the cast links arms and sings ''Goodnight, Sweetheart,'' the Plymouth overflows with good feeling.