On a Wednesday night in the wings of the West German Embassy's Carl Schurz Auditorium, 60-year-old Mary Jefferson, community-christened blues maven, shakes, shimmies and slowly rolls her hips. She's cheerleading. "All right, all right," she yells in a sweet, raspy voice, "lettum play."

The occasion is a cross-cultural exchange between the Young People's Jazz Orchestra from Dortmund, West Germany, and the LETTUMPLAY Jazz Ensemble of Washington, D.C. LETTUMPLAY is the arts organization dedicated to the preservation of jazz, founded by the late Tony Taylor in 1976. On this night its constantly changing ensemble is a handful of musicians in their twenties: Lewis Cottam Jr. on guitar, Tony Addison on drums, David Marsh on bass, Frankie Addison on saxophone and Michael Wade on trombone and trumpet.

The ensemble smoothly shifts speeds during its version of Miles Davis' "Seven Steps to Heaven." Jefferson is ecstatic. "This is what LETTUMPLAY is all about," she says, motioning over the audience, which includes West German Ambassador Gunther van Well and his wife Carolyn, embassy officials and families, local German clubs and a sprinkling of local jazz supporters. "Everyone understands music." In a moment the audience will join the 17-member German Jazz Orchestra and its conductor, Rainer Glen Buschmann, in a wavelike response to Jefferson's show-stopping version of Ma Rainey's "C. C. Rider."

"Mary has what the Germans call Menschkenner -- an understanding of human nature," says Paul Logan, chairman of Howard University's German and Russian Department and co-organizer of the event. In the spirit of the moment, Logan is discussing the possibility of a future U.S. Information Agency-sponsored visit to West Germany by the LETTUMPLAY Jazz Ensemble. "As you can see here tonight," he continues, "music breaks down all barriers."

Two nights later, LETTUMPLAY is performing at the Anacostia Men's Shelter run by the Coalition for the Homeless. This time LETTUMPLAY is Bill Washburn, drums; Barry Tymas, guitar; and Danny Ellis, vocals, joining guitarist Cottam. By the time the band has run through its first number, "Girl From Ipanema," the crowd of men and visiting women from the Florida Avenue Shelter are tapping their feet on the cafeteria floor, and their weathered hands are finding the rhythm.

LETTUMPLAY's mission, Jefferson says, is to teach aspiring young musicians technical skills so they'll be better equipped for careers in music. Over the last seven years, about 100 young Washington musicians have progressed through LETTUMPLAY as part of the Mayor's Summer Youth Employment Program and Arts D.C. The seven-week program includes rehearsals and expanding repertoire as well as live performances.

Former members of the LETTUMPLAY Youth Ensemble include local success stories such as saxophonist Marshall Keys, currently at the newly opened Oxford Club in Georgetown, and Chuck Royal, a music teacher at the Duke Ellington School of the Performing Arts. Royal also has two albums to his credit.

According to Jefferson, LETTUMPLAY has another clear objective: "Our organization carries the music and the history of the composer of America's only classical art form into the schools, churches and auditoriums, and to the sometimes forgotten," she says.

Since 1979, LETTUMPLAY has performed free concerts at such places as Children's Hospital, Lorton Reformatory and the Veterans Administration Hospital. The group also has been featured in music festivals at the Fort Dupont Summer Theater with Jazz Messenger Art Blakey, and at Carter Barron Amphitheatre with vocalists Nancy Wilson and Betty Carter. From 12:30 to 3 p.m. Sunday LETTUMPLAY will perform at Catholic Charities, 2800 Otis St. NE, to benefit Hands Across America, the nationwide effort to raise money to help America's hungry and homeless.

Most of LETTUMPLAY's $78,000 budget comes from local grants, including a $2,500 D.C. Arts and Humanities individual grant to Jefferson, which she has applied to LETTUMPLAY. According to Arts D.C. Executive Director Lynne Zamil, LETTUMPLAY'S "excellent quality" is being rewarded this year with a stipend to cover overhead for the upcoming summer youth program.

"It's a LETTUMPLAY family," says drummer Addison, "and Mary Jefferson's the mother of LETTUMPLAY.

"LETTUMPLAY's not just the musical training," he says, "but it's understanding the jazz culture too, like who gave us the music we're playing today."

At the West German Embassy performance, the Jazz Ensemble musicians are moving slowly into T-Bone Walker's "Stormy Monday," their last scheduled number for the evening. They're working to find the right A progression. "Mother" Mary Jefferson sits at attention, watching.

"I worry over my babies like a mother hen," she says, closing her eyes, concentrating so they'll get it right. Soon the ensemble finds its pace, guided by Cottam's guitar. Jefferson opens her eyes and smiles, "All right," she whispers, "lettum play."