ONE AFTERNOON in the 1930s, composer and music critic Deems Taylor had lunch with George Gershwin, planning to go on to a baseball game afterward.

"During lunch," Taylor recalled later, "he remarked that he was just finishing an opera about Negro life in the South.

""A what?" I said.

""You heard me."

"So back to his house we went, and he played me all four hours of the score of "Porgy and Bess." He was right. It was an opera -- and we never did get to the ball game."

Taylor heard history being made on Gershwin's piano that afternoon -- a new bridge spanning the historic gap between opera and the Broadway musical. Gershwin was not the first American to cross that bridge; Scott Joplin and Victor Herbert had done it before him. But Joplin died unrecognized, and Herbert did it with a kind of music that was really European in style and form. Gershwin was the first to do it "successfully" with American idioms.

Long before composing "Porgy and Bess," Gershwin sharpened his skills with "Lady Be Good," "Oh, Kay" and "Funny Face." That background is evident both in the music and in some of the places where it is performed. Before its four-week run in Washington, which opens Wednesday evening at the Kennedy Center Opera House, the latest production of "Porgy and Bess" spent two months at the Radio City Music Hall in New York -- a place that is not usually listed among the world's leading opera houses -- and after it finishes its American tour, there are plans to send it to La Scala in Milan -- a place that is.

From the Music Hall to La Scala is an enormous distance. Gershwin made a similar leap nearly half a century ago when he broke out of his Tin Pan Alley background to play in the same league with Bizet and Puccini. And in recent years, American music has been moving the same way: the Broadway musical is taking on the qualities of a classic art form, while opera is adopting values that used to be identified with Broadway.

It happened first in "Porgy and Bess," a turning point in American music history as well as a major work of art. Gershwin himself was not always sure what to call it, in spite of the cocky air he assumed when he was talking to Demms Taylor. Is "Porgy" a Broadway musical, as indicated by its original production (on Broadway, under the auspices of the Theater Guild), by its frequent use of syncopation and by the background of its composer, a high school dropout who studied classical piano and theory but made his living as a pop musician? Or is it an opera, as indicated by its use of recitatives, the repeated reliance on a seven-note leitmotif ("Bess, You Is My Woman Now"), the clearly classical aspirations of most of its music and the need for operatically trained voices in its leading roles?

Gershwin finally decided to call it a "folk opera," evidently hoping to capture both the popularity of a Broadway show and the prestige of classical music. It did not work; classical critics (Olin Downes and Winthrop Sargeant among them) panned it on opening night. It ran for 124 performances -- high by operatic standards but close to failure in Broadway terms.

The critics were wrong; "Porgy and Bess" is an opera, even with the jazz piano that can be heard as the curtain rises and such pop-flavored vocal numbers as "It Ain't Necessarily So." Gershwin justified his "folk" tag for the work abundantly, with real folk material and very close imitations: the cries of street vendors he heard in South Carolina, work songs and religious music. Some of the material is in a jazz-flavored pop style ("A Woman Is a Sometime Thing," for example), and even the most purely classical numbers, such as "Summertime" and "Bess, You Is My Woman Now," are given their distinctive character by blue notes in the harmonies and the melodic cadences. But all this does not mean that Gershwin departed from classical style; what he did was enlarge it.

The opera's ancestry includes "Show Boat," but goes far beyond that pioneering model of the "book musical," which finally displaced the Victor Herbert-style operetta (based on European models). "Porgy and Bess" has nothing to do with operetta, although it has moments of high and low comedy. It also has nothing to do with "musical comedy," a term developed for lack of any better name and applied inappropriately to works like "West Side Story" and "Sweeney Todd," two distinguished descendants of "Porgy and Bess." All these "musical comedies" are essentially tragic, but even today the standard terminology has trouble with tragedies involving common people. Traditionally, tragedy is something that happens only to members of the upper classes.

That class distinction in opera was severely bruised by Giuseppe Verdi in "La Traviata," but it was not really shattered until the arrival of the verismo style near the end of the 19th century, with works like "Cavalleria Rusticana" and "I Pagliacci," telling tragic stories about members of the lower classes. These operas and "Carmen" (which was originally classified as an opera comique) represent the tradition in which Gershwin was trying to write, and he succeeded splendidly, giving verismo an American accent -- musically and dramatically.

It is hard to imagine the original impact of this work, hundreds of productions and thousands of performances later, when it has become established as a classic, a cornerstone of American opera. But it must have been a shock on Oct. 10, 1935, when "Porgy and Bess" was heard for the first time.

Except for insignificant walk-on roles (which account for almost all the spoken dialogue in the opera and none of the music), the cast was all black. This had been done before in Virgil Thomson's (and Gertrude Stein's) "Four Saints in Three Acts," but that was a bit of dada that did not have to be taken seriously. "Porgy and Bess" was a work that demanded to be taken seriously; it was about real people and all of them were black, and they were singing in English so that the audience could know exactly what was going on.

Opera in English was hard enough to accept; everyone knew that operas were written in Italian, German or French, and there were rumors that one or two had been written in Russian. But an opera about black people, in English -- a sort of black English at that -- with traces of French Impressionism in the orchestration, some songs that were clearly in a jazz idiom and others in the musical language of Puccini -- that was entirely too much. It is a wonder "Porgy and Bess" survived for 124 performances. If the composer had not been Gershwin, it might have folded much sooner. But if the composer had not been Gershwin, of course, it would not have been "Porgy and Bess."

It took a while for the classical descendants of "Porgy and Bess" to come into view, but they are all over the landscape today and some have been seen on television, helping to build an audience for American operas in American idioms. Carlisle Floyd's "Willie Stark" is one and Lee Hioby's "Summer and Smoke" is another; they are two names among hundreds. Another descendant, embodying the classical-popular ambiguity of "Porgy and Bess," is Leonard Bernstein's "Candide," which opened a run last week at Arena Stage.

Like "Porgy and Bess," these new American operas embody solid theatrical and dramatic values. They are in the language of the audience, which means that the composer cannot get away with a mediocre libretto, and they are about people with whom the audience can readily identify despite social and economic differences. Their music is rooted in the styles of this continent as well as the great traditions of Europe. The craftsmanship required for their production meets the standards of Broadway -- as American opera seldom did before the arrival of "Porgy and Bess." And their appeal is less a matter of snobbery and more a matter of spontaneous enjoyment.

When "Porgy" first burst on the scene, American opera was still a sometime thing. Nearly half a century later, the curtain is still going up.