 Kelly was much in demand as a sideman for recordings, and appeared on albums by most of the major jazz leaders in the late 1950s and early 1960s. In April 1957, for instance, he appeared as a guest in an enlarged version of Art Blakey's Jazz Messengers, for an album later released as Theory of Art; this band included trumpeter Lee Morgan, with whom Kelly had recorded a few weeks earlier. The recording sessions continued four days later, with Kelly joining Blakey, Morgan and others on Griffin's A Blowin 'Session; this was followed by three studio days for Gillespie, and another for trumpeter Clark Terry, before the end of the month. Later that year, Kelly made a rare appearance playing bass, for one track of vocalist Abbey Lincoln's That's Him!, after the regular bassist, Paul Chambers, became drunk and fell asleep in the studio.
