OKT4+ (T helper/inducer) and OKT8+ (T cytotoxic/suppressor) subsets were depleted from peripheral blood lymphocytes (PBL) by complement-mediated lysis and residual cells examined for responsiveness to pokeweed mitogen (PWM) using a protein A haemolytic plaque assay for immunoglobulin secreting B cells. It was shown that: (1) three cycles of cell killing were required to totally abolish T helper function; (2) OKT4- PBL did not respond to PWM, but in a co-culture system, an equal number of unfractionated normal PBL could entirely reconstitute responsiveness of the residual B cells; (3) OKT8- PBL gave enhanced numbers of PWM-induced plaque forming cells (PFC); (4) addition of 4 micrograms/ml concanavalin A (con A) to PWM stimulated OKT8- PBL failed to suppress PFC generation, but suppression was induced by 12.5 and 25 micrograms/ml con A and (5) kinetics of PWM-induced PFC development were similar in the presence or absence of OKT8+ cells.