Last night at the Library of Congress there was -- incredible as it may seem -- the American debut of a musical by George and Ira Gershwin.

The good news is that s'wonderful.

Called "Primrose," it is no mere curiosity whose only claim to fame is its supremely distinguished pedigree. It is mature Gershwin, coming from 1924, the same year as no less a work than "Rhapsody in Blue." The music is spirited, and considerably less jazzy than most of Gershwin's show music. Its ballads are distinctly Kern-ish, and it is full of patter songs that sound quite English, which is not inappropriate because the musical was composed for staging in England, where it is said to have been a great success.

But what is truly top-drawer about "Primrose" are Ira Gershwin's resplendent, droll and gloriously imaginative lyrics. His wit and his genius for rhyming are at a sort of peak. How about the superb patter song that brings the first act to an end? It is called "The Mophams," and it is a marvelous parody of the British upper crust, in this case a family suitably named Mopham. How's this for rhyme: "A Mopham is always a Mopham; a great race the Mophams; the danger is that no one can stop 'em."

The other work on the library's delightful all-Gershwin program, in which songs from two musicals were performed in concert by splendid singers, is better known: "Pardon My English," which is from 1933, only two years before the Gershwins' masterpiece, "Porgy and Bess."

It was the first time in many years that the songs from "Pardon My English" were performed in their original, and long lost, orchestration. All of "Primrose" had been lost as well. It finally surfaced when it was discovered, along with many other works by the Gershwins, Kern, Rodgers and Porter at a Secaucus, N.J., music warehouse in 1982.

Apparently "Primrose" was deemed too British for Broadway. In ways it brings to mind Noel Coward, both musically and dramatically, except that Coward would never have written so awkward and inane a book as the one done for the Gershwins' by George Grossmith and Guy Bolton. Desmond Carter joined Ira in writing the lyrics.

The premise goes like this, according to the program's plot summary: "At the moment Hilary is engaged on a serial, entitled 'Primrose,' of which his fair young neighbor, Joan, is an enthusiastic reader; indeed she assumes that the novelist has taken her for his model in the title role."

The story of "Pardon My English," by Herbert Fields, is not much more lucid. It's set in Dresden, and the German government has outlawed soft drinks. So soft drink speakeasies are proliferating.

So both works are better off in concert form, where music and lyrics more than carry them. It may be a cliche, but the truth is that no one is writing stuff this good these days -- "Les Miserables" very much included.

"Pardon My English" has a few classics, such as the delightful "Freud and Jung and Adler," followed by a delectable one, "He's Oversexed." (Another standout: a great parody sung by a police commissioner, "No Tickee, No Washee."

There were six singers, and most handled more than one role. The clear audience favorite was the lusty mezzo Kim Criswell, who has a fine voice and sings with relish.

George Dvorsky was a stylish high tenor. Cris Groenendaal showed excellent control and diction. John Sinclair was very strong in the buffo roles. Rebecca Luker was a good high soprano. And Jack Dabdoub was an able police commissioner.

John McGlinn conducted the Musiccrafters orchestra and the Norman Scribner Choir ardently.