Fifty women strolled into the recreation room of Washington's new jail for a jazz show Thursday afternoon, all wearing identical faded blue denims cinched by elastic waistbands.

Each woman had done her best to personalize the prison garb: Nicknames like "Blondie" and "Short Stuff" swirled in black magic marker; cuffs were torn into fringes; collars were twisted this way and that. And for this special concert, there were freshly braided cornrows, tightly tied red scarves, and bright silver earrings.

When Calvin Jones, the leader of the U.D.C. Jazz Septet, went into a Count Basie-like piano solo, the women started clapping and swaying in their plastic seats to the beat. When the legendary tap dancer, Mr. Rhythm, stepped onto a section of panelling taped to the carpet, the women crowded to the front to watch his 62-year-old feet clack like a typewriter. And when singer Mary Jefferson finished off "Green Dolphin Street" with an Ella Fitzgerald flourish, the women jumped right out of their seats.

"No jazz show would be complete without the blues," Jefferson crowed. "They say the fellas get them more than we do . . ." Before she could finish, inmate Laverne Gordon shouted emphatically: "Oh-h-h-h no, they don't!" There were vigorous nods all around the room.

The concert was one of five this past week in the "Jazz for the Sometimes Forgotten" series sponsored by Lettumplay -- a local arts organization which brings live jazz shows to people in institutions. It is funded by the National Endowment for the Arts and the D.C. Commission on the Arts.

The line-up of the U.D.C. Jazz Septet, Mr. Rhythm, and Mary Jefferson performed for the women at the District of Columbia Corrections Detention Facility (the "new jail" to the inmates), for the maximum security prisoners at the Lorton Reformatory Tuesday, for the senior citizens at D.C. Village Monday and at St. Mary's Court Friday.

"We'd be looking at TV right now if it weren't for them," inmate Michelle Fox confided Thursday during one song. "We really don't have much to do. We stay locked up 17 1/2 hours a day. So when someone out there does care enough to come and do something, we show them we appreciate it."

Jefferson's voice was reaching further and further into her throat for the gutbucket blues tones of Bessie Smith. "The blues ain't nothin' but a woman," Jefferson wailed, "Wanting another woman's man/She can't get him when she wants to/She has to get him when she can." Several women squealed out in recognition; one fanned her face to cool off. "You see," Laverne Gordon whispered, "it lets us get our frustrations out."

"You're making me sweat," one woman called out to emcee Tony Taylor. "You're making me sweat too," Taylor grinned back. Taylor founded Lettumplay in 1977. He is still the group's director as well as assistant to the chairman of the D.C. Arts Commission. Lettumplay has sponsored music seminars, jam sessions, free lunchtime concerts and these shows for the shut-ins. But long ago, before he joined these non-profit groups, Taylor founded the Bohemian Caverns, Washington's fabled jazz nightclub.

"I danced for Duke, Pearl and Louie," boasts Mr. Rhythm of his work for jazz legends Ellington, Bailey and Armstrong. Known offstage as Carl Jackson, Mr. Rhythm has been tap dancing for almost half a century since he learned the steps as a teenager on the streets of Washington.

Taylor helped Mr. Rhythm get a grant from the D.C. Arts Commission for these five shows and 17 others at area senior citizen centers. He will also give recitals at Jan Van Dyke's Dance Studio Feb. 16-17. The aged, rotund figure in the blue beret moves slowly till he hits the stage. Then the feet go into a blur of glides and taps. "Just keep movin', man," he chuckles by way of explanation.

Mary Jefferson has been singing in Washington for 39 years, since the age of 14. She sang with the Gene Ammons and John Malachi bands and often appeared at Howard Theatre and Bohemian Caverns."Audiences in these institutions are much more responsive," she claims. "There's almost a spiritual electricity passing. Where else do you get standing ovation's? They give out so much and you want to give even more."

"I like the senior citizen places best," notes trombonist Clinton Hyson. "You don't expect them to have too much energy, but they always do. At one place last year, they got up and into a line and started dancing around doing the bump. One lady threw down her cane and said, "I don't need it today. I don't need my medicine today either. You're my medicine today.'"