BOSTON -- Those who have never entered the inner sanctum of an all-male club are likely to envision it as the exclusive, wood-paneled retreat of the elite. There the very late George Apley and his cronies, resplendent in tweeds, still sit in leather wing chairs, puffing pipes, rising only occasionally to toast good fellowship and lament the loss of the gold standard.

But this vintage 19th-century scene has been updated. On any given day, such a club is not just hosting George and his unreconstructed pals. It's probably serving breakfast to the law firm of Biddle, Biddle and Fiddle, lunch to the monthly meeting of the investment bankers and dinner to the state insurance association.

In the twilight of the 20th century, the most vaunted private clubs are also in the business of business. The women excluded from membership or banned from the premises -- women who aren't allowed to sit in the lobby or walk through the front door -- are penalized in doing business with the boys.

So, today, the Supreme Court is going to hear what might be called The Last Bastion Case. It is listed formally as New York State Club Association v. City of New York. It will test the constitutionality of a law, already copied in at least half a dozen other cities, that would force these clubs to choose -- gasp -- between admitting women or giving up money.

This choice is not just an arbitrary one: your money or your manhood. Under the Constitution, any group of Americans has the right to private association. If the blue-eyed, right-handed bird lovers of Wisconsin want to form an exclusive club for the purposes of warbling, the government cannot force them to open up the premises to brown-eyed, left-handed cat fanciers.

More to the point, any group of males, young boys or old boys, can freely build its plywood clubhouse and post a sign: NO GIRLS ADMITTED. Any group of girls can do the same thing. But if a private club gets involved in public, commercial activity, how long can it go on claiming the rights of ''private association''?

The New York law says that a club stops being private and starts being subject to public laws -- including those against discrimination -- if it has more than 400 members, provides regular meal service and gets regular money from nonmembers ''in furtherance of business or trade.'' Most of the rich and famous clubs do just that.

In 1980, 37 percent of city-club income and 26 percent of country-club income in America came from memberships that were paid by businesses. Companies paid men to belong to clubs that barred their female colleagues. It's estimated that 85 percent of the money spent at these same clubs was treated as business expense, so that taxpayers also supported ''private clubs.''

Columbia Law Prof. Jack Greenberg, author of the law, made this careful distinction: ''If it's a club where people hang out, are affable and drink port it's not covered.'' But if it's the Century Club and assorted other last bastions of New York business, it is.

This case goes to the Supreme Court at something of a historic moment. Justice Blackmun recently resigned from one all-male club. The brand new Justice Kennedy resigned from another all-male club. A third justice, Sandra Day O'Connor, was once barred from an all-male club.

It was Justice O'Connor who wrote the decision that opened up the Jaycees and Rotary: ''When a club enters the marketplace of commerce in any substantial degree, it loses the complete control over its membership that it would otherwise enjoy if it confined its affairs to the marketplace of ideas.'' Do I hear a bell knelling over the Century Club door?

The issue of integrating clubs is often seen as one of those elitist matters of importance only to a handful of already privileged women such as the late George Apley's granddaughter, a Princeton graduate and corporate lawyer. But as any woman who has ever been excluded from a lunch meeting or forced to go through a side door to join her companions can tell you, these clubs are a real part of the fabric of business in America.

In that sense, this is not an attack on single-sex associations, the kind many of us choose for friendship. There are all-male and all-female environments, segregated retreats, coffee klatches and clubs, that sustain many of us. But business is not private, and professions are not an all-male club. And clubs that pocket profits from business cannot run for the protection of privacy. The law before the Supreme Court says simply: they can't have it both ways anymore.