In "First Monday in October" Robert E. Lee and Jerome Lawrence have achieved a playwriting coup - a play about the Supreme Court which is serious and trenchant and, at the same time, funny and richly human.

It provides gorgeous, sharp roles for Henry Fonda and Jane Alexander, who act them with the esprit of skilled players relishing their conflicts.

"First Monday," which is at the Eisenhower Theater unitl Feb. 25, is about what happens after a busy president appoints the first female Supreme Court Justices. Lee and Lawrence, who took their title from the Court's traditional fall opening, set out to humanize those black-robed justices in their struggle with law's eternal battle between doubt and sincerity.

Ruth Loomis, widow of her law partner and still young enough to make tennis a habit, joins the Court to the dismay of irascible, liberal Justice Daniel Snow, who considers the conservative newcomer "the Lysol of Orange County." As the last-appointed of the nine, Loomis also is the last to enter or leave any room.

The two are natutral antagonists, alike in their tireless determinations and different in their attitudes about property and human rights. She is neat. He is messy. She honors the memory of her dead husband and partner. He takes for granted his living wife.

Lee and Lawrence have arranged several cases for their bench conflicts - a pornographic movie which he refuses to see because "no matter how terrible it is, it's covered by the First Amendment," and a case involving a multi-national conglomerate. The cases are presented fairly and clearly. Where does doubt begin and certainty end?

In such plays as "Inherit the Wind" and "The Night Thoreau Spent in Jail," these playrights leavened their passion for justice with amusing human vignettes, just as in "Auntie Mame" and "Mame," ostensibly frivolous, they slipped in the serious. Here, though their root purpose is serious, they have their fun.

We watch, for instance, the formation for that picture Americans get every October of the Court seated, solemn and sated. When the Court photographer (John Stewart) monkeys with his camera, we get the impatient humans under the robes, mocking their formality but knowing its value.

And we see them restlessly watching movie porn in a scene of high funny mime.

There is amusement, too, in watching Loomis and Snow battle each other, both becoming the sworn witness in the chair. It is the lawyers' game of Dvil's Advocate played with high relish.

Fonda's simplicity disarms his art. The Fonda voice, tones and gestures are there, but they are Snow's. It is an errie thing to watch a familiar, even loved image remain itself while becoming quite another person.

That acting is a deceptive art also is evident in Alexander's firm, steely Ruth. Here the actress looks wholly unlike the Eleanor Roosevelt she played on TV or one of "The Three Sisters" she played at Arena Stage.

The Alexander self is wholly effaced. Ruth becomes a very female female but a wholly asexual brain. Only when the script reveals a relevant fact about the past does her armor allow a chink, and then only a very slight one.

There is about "First Monday in October" a welcome, almost rare professionalism. There is Larry Gates to act the astute, perceptive Chief Justice, a performance so warmly natural that Warren Burger would find him welcome.

In the other 13 parts there is nice work by Tom Stech Schulte as the Snow clerk and John Wardwell, as a scrappy justice in the photographic session.

Oliver Smith's sliding sets are smooth and suitably elegant and director Edwin Sherin's guidance is imaginative and suave.

The temptation, after Tuesday night's performance, was to spill some of the funny lines - especially for lawyers, news types and true Washingtonians. But let it at least be noted that Fonda's line about the roof leaking has nothing to do with the Kennedy Center's present state. If you keep listening through the laughter, you'll learn he's really referring to the Temple of Karnak.