The present study demonstrates the minimal, optimal, and synergistic signals involved in the activation of normal human peripheral blood and tonsillar B cells to proliferation. Initial activation signals were delivered to B cells by low concentrations of anti-mu antibody which did not induce proliferation by themselves. However, marked synergy was seen when anti-mu antibody was added to cultures in the presence of monoclonal B cell growth factor (BCGF) obtained from a human T-T cell hybrid such that the B cells underwent substantial proliferation. This latter proliferation was seen without maturation into Ig-secreting cells, which indicates that the BCGF is not a differentiation signal but a signal that drives the cell up to but not beyond the proliferative phase. Of note was the fact that B cells reflected differential sensitivity on the basis of size to either the activation signal delivered by anti-mu antibody or the proliferative signal delivered by BCGF. BCGF directly stimulated the larger B cells in the normal tonsillar B cell repertoire to proliferate without the requirement for an in vitro activation signal, which indicates that the cells had already received some form of activation signal in vivo. Indeed, these cells expressed the 4F2 antigen found on activated but not resting lymphocytes. In contrast, the smaller tonsillar B lymphocytes did not express the 4F2 activation antigen and required activation by anti-mu antibody, which did not of itself induce proliferation, but which acted in synergy with BCGF for substantial proliferation of the B cells. These studies thus provide a useful model of human B cell activation, proliferation, and differentiation and allow a more precise delineation of each phase in this cascade.