George Furth has idententified a wrong, and moved bodly to right it. With "The Supporting Cast," which opened last night at the Eisenhower Theater, Furth has done as much as any one playwright could possibly do by means of any one play to solve the underemployment crisis among mature stage actresses.

There are only five parts in "The Supporting Cast," but four of them have gone to Hope Lange, Sandy Dennis, Betty Garrett and Joyce Van Patten, estimable actresses all. A theater that cannot regularly exploit such talents should be ashamed of itself, and it follows that Furth and his producers should be saluted for their attempt to correct the situation. But if a long-term solution is what they're after, they may be disappointed. As of last night, their effort looked like a make-work program that would never stand up to strict cost-benefit analysis.

It's not that the Misses Lange, Dennis, Garrett and Van Patten fail to entertain. It's just that the entertainment is a very strained and shallow kind, even by the current standards of Broadway comedy. An idea of lurking -- an idea that must have looked sharp when the author hatched it. "The Supporting Cast" is a comedy about one of those potboiling novels in which real and famous people appear in thin fictional disguises. Ellen, played by Hope Lange, has written such a novel, and the other characters are characters in it. A few days before the publication date, Ellen invites them to lunch at her Malibu beach house in order to explain the basic good will and noble purpose that lie beneath a story that (she accurately fears) will appall all the prototypes who figure in it.

They have something in common beyond their presence in Ellen's book. All of them (and that includes Jack Gilford, the only male in the cast) are intimately related to famous peole -- a movie star (Van Patten's husband), a glamorous congressman (Dennis'), a musician (Garrett's son) and a playwright (Gilford's wife).Ellen herself has lived in three shadows: athose of her politician-father, a golfer first husband and a professor-author second husband.

Like Clare Boothe Luce's "The Women," "The Supporting Cast" leaves the principal players out of the action in order to focus exclusively on the subordinates. And like "The Women," too, it ultimately dispenses the calming moral that there is heroism as well as pain in playing second fiddle.

It's no crime to borrow a formula from an old play, of course. The problem is that Furth hasn't borrowed enough. "The Women" was a veritable sardine can of melodrama, but "The Supporting Cast" is so loosely packed with plot that its characters have time to engage in the same labored jokes over and over again.

On the character front, more problems loom. Ellen, the novelist, is a completely vacuous central character who does nothing but dispense pinheaded wisdom and plead with people to finish her book before they jump to conclusions. Lange can do nothing in the role but look nice (she does) and know her lines (she's well on the way).

At the other end of the personality spectrum lies Sally, the congressman's neurotic wife, whom the playwright has equipped with a grab-bag full of dizzy eccentricities -- a fear of heights so intense, for example, that she can't tolerate the second-floor balcony of a beach house.

Sandy Dennis gives a performance as hectic as the characterization. It's a shame, because Dennis has funny moments, and there is an essentially funny idea in a woman who claims to have recovered from a mental breakdown which she is, very clearly, still in the midst of.

THE SUPPORTING CAST by George Furth; directed by Gene Saks; scenery by William Ritman; costumes by Jane Greenwood; lighting by Richard Nelson; with Sandy Dennis, Hope Lange, Jack Gilford, Betty Garrett and Joyce Van Patten.

At the Eisenhower Theater through July 25.