LIFE photographer Gjon Mili (who also directed the classic 1944 short film, Jammin' the Blues) often hosted jam sessions at his photography studio in New York during the 1940s. The pictures in this gallery testify to the talent on hand — both musical and photographic—at those all-night parties. Duke Ellington, Billie Holiday, Dizzy Gillespie, Count Basie, Lester Young, Roy Eldridge, Cozy Cole, Gene Krupa . . . like the jam sessions themselves, the names of the greats who played at Mili's studio go on and on and on.
Born in Albania, raised in Romania, Mili emigrated to America to study electrical engineering at M.I.T. Inspired, in 1937, by M.I.T.'s Harold Edgerton's development of the stroboscopic light, Mili went on to experiment with strobes, film speeds, unusual compositions and subjects—in short, he applied his prodigious technical prowess and dedicated his artist's eye to new ways of seeing.
Time, he realized, "could truly be made to stand still. Texture could be retained despite sudden violent movement." These insights, combined with his love of jazz, helped him create some of the most intimate, unique portraits of jazz legends ever made by any photographer—all in what LIFE magazine called his "smoky sweaty barn of a studio."
As for the jam sessions themselves, LIFE (helpfully) wrote in its Oct. 11, 1943, issue in which some of these pictures first appeared:
A jam session is an informal gathering of temperamentally congenial jazz musicians who play unrehearsed and unscored music for their own enjoyment. It usually takes place in the early morning hours after the participants have finished their regular evening's work with large bands. . . . It represents the discarding of the shackles imposed by working with a band that plays You'll Never Know and All or Nothing at All in the same unimaginative arrangements night after night. It represents the final freedom of musical expression.
Recently such a session took place in the New York studio of LIFE photographer Gjon Mili. From shortly before 9 p.m. until after 4 a.m. some of the most distinguished talents in jazz performed for an audience which, in the smoky sweaty barn of a studio, derived an alert, fascinated, almost frenzied enjoyment from what it heard.
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter at @LizabethRonk.