GROUCHO! - At Ford's Theater through June 3.

If you had a friend who did a really good Groucho Marx imitation, you would pester him to do a few classic jokes, the funny walk, the bit with the eyes, and the leer. Then you would expect him to stop, after everyone had gotten the idea.

Lewis J. Stadlen does a good imitation, but the question is why spend a whole evening doing it, in "Groucho!" at Ford's Theater, and call it a play.

Of all the popular cultural and political figures who are the subjects of these one-man exposition plays - which make politicians, musicians, writers and humorists of different periods and countries appear less like important individuals than a brotherhood of celebrities - there is the least reason to do one of Groucho Marx.

There's nothing particularly fascinating or instructive in the well-known biographical material covered by this show. The Marx Brothers' mother's contribution has been previously dramatized to make a dramatic point, but Groucho's mere recital of the family's deserved success doesn't even have the novelty of gossip. His pathetic old age, which could conceivably be used to make an ironic point, is merely represented by having him recite a number of tedious, self-aggrandizing and otherwise pointless stores while his hand shakes - a well-done acting exercise but hardly a drama.

The best reason for not bothering with this effort is that there is no danger of our losing Groucho Marx's contribution to American humor. Why see a good Groucho imitation, when you can see Groucho himself on film?

For this reason, the side-kick role, also well done here, is more interesting. Nancy Evers does a combination of accompanist, the snooty-lady role, the epitomy of Culture, the "You Bet Your Life" guest and general straight-woman that is very amusing.

And if you want a compact Groucho anthology, this one contains the basics: the joke about not wanting to join a club that would take a member like him, a rendition of "Lydia, the Tattooed Lady," a coat tossed into the audience with the admonition, "Would you please clean that and have it back to me by Friday?" CAPTION: Picture, LEWIS J. STADLEN IN "GROUCHO!" By Harry Naltchayan.