We have developed a microculture system suitable for limiting dilution analysis of Epstein-Barr virus (EBV)- and pokeweed mitogen (PWM)-induced activation of immunoglobulin secretion by human B cells. It was found that exogenous filler cells were not required to obtain optimal EBV-induced B cell precursor frequency (PF) estimates, although filler T cells were required for optimal PWM activation. In fact, when autologous T cells were used as filler cells, a marked decrease in the EBV-induced IgM PF was noted. Treatment of the T cells with cyclosporin A partially eliminated, and irradiation of the T cells completely eliminated, this decrease. The calculated PF of B cells activated by EBV was from 1/290 to 1/3,700 for IgM, and from 1/920 to 1/3,250 for IgG secretion. PWM activated from 1/140 to 1/3,200 B cells to IgM secretion. The results of experiments in which EBV and PWM were mixed, indicated that these two polyclonal activators operated on different B cell subpopulations. Therefore, both these agents seem to activate small, discrete subpopulations of human peripheral blood B cells to Ig secretion.