The last new piece of the Stuttgart Ballet's current Kennedy Center season is as bright as something to C major music by Mozart ought to be. It bears the score's name - "Concerto for Flute and Harp" (K 299), and has an unusually constituted cast. There are two ballerinas but the corps are all men and all the men are corps, for even the women's partners emerge from and on occassion rejoin the men's group.

John Cranko, who choreographed the work in 1966, was not just alluding to the music with this distribution of roles. While the ballerinas represent the solo instruments and the men the orchestra as a whole, he wanted to have fun by reversing the traditional sex ratio of "white" ballets. And, at first, "Flute and Harp" indeed seems as if it will be a great deal of fun.

The men are in two lines that play together or against each other. Sometimes big and joyous steps - turns, beats, leaps - pass up and down the lines like waves. After a while, though, the novelty flags. Despite some quicksilver 18th-century ornamentation, the 19th-century ballet technique begins to look too pretty for the music. The piece never becomes stale, but it just is not one of the great Mozart ballets.

The program, which also include Jiri Kylian's "Return to the Strange Land" and Cranko's "Initials R.B.M.E.," gave the audience another chance to look at the Stuttgarters as pure dancers instead of as dramatic dancers or mimes.

They are not, as a company, light. Nor is their solidity sensual and dynamic. Jean Allenby, ho was seen in the Kylian ballet, is an exception. She has an earthy flow reminiscent of the old Ballet-Russe style. There is drive to her dancing and she can use gravity as a positive quality.

Barry Ingham, one of the lead men in the Mozart, has suppleness but not sufficient dynamics. Lucia Montagon, appearing in "Initials," was back after an injury. She stands out, on occassion, because she can move in that elegantly easy, whole-bodied way of Soviet dancers.