The functional role of an adherent cell in assisting a variety of in vitro immune responses is well established. An assay for human monocyte chemotaxis in vitro was utilized as a means of investigating the role of adherent cells in the production of the monocyte chemotactic factor that is produced by peripheral blood lymphocytes upon stimulation with Concanavalin A. Depletion of a population of adherent cells by passage of peripheral blood mononuclear cells through a Sephadex G-10 column rendered the latter incapable of producing monocyte chemotactic factor. The requirement for adherent cells in the production of a "lymphokine" is in agreement with many previous works of a similar nature performed in other experimental systems.