A recently described system for the induction and assay of plaque-forming cells (PFC) following polyclonal stimulation of human B lymphocytes was employed to delineate the cellular requirements for pokeweed mitogen (PWM)-induced activation of human tonsillar and peripheral blood B lymphocytes. It was found that the process was not dependent on the presence of monocytes, but was dependent on the presence of T cells. T cell depleted suspensions of tonsillar as well as peripheral blood lymphocytes had markedly diminished if not absent PFC responses following PWM stimulation. The PFC responses of T cell depleted cultures of tonsil and blood could be reconstituted by adding back T cells. T cell supernatants could also reconstitute the PFC response of T cell depleted tonsillar lymphocytes but not of T cell depleted blood lymphocytes, thus suggesting a more stringent T cell requirement of the PWM-induced PFC response of blood lymphocytes. Enrichment for T cells on the other hand enhanced PFC responses in both organs, particularly in those cultures which had low responses in unfractionated suspensions demonstrating the requirement of optimal T cell help for maximal PFC responses. This study demonstrates the feasibility and effectiveness of this PFC system in elucidating some of the complex regulatory processes associated with the activation of human B lymphocytes.