Using a culture system that includes a B-enriched fraction of peripheral blood lymphocytes (PBL) mixed with non-proliferating "filler" cells (mitomycin-treated PBL or T cells), the kinetics of human B cell proliferation and differentiation into cells containing cytoplasmic immunoglobulin (cIg) have studied. Following stimulation with phytohaemagglutinin (PHA) or pokeweed mitogen (PWM), proliferative responses of enriched B cells were comparable to those for unseparated PBL, with a peak at 3 to 5 days and a steep decline thereafter; cIg-positive cells increased with both mitogens from day 3 until 7. Little proliferation or differentiation occurred in B cell cultures lacking T "filler" cells. The results indicate that human B cells can respond polyclonally to PHA in vitro, as well as to PWM, if non-proliferating T cell "help" is present. Further, these proliferative responses, but not subsequent differentiation, were inhibited by PHA-stimulated blasts that were treated with mitomycin and added on day 3 of culture.