The activation, growth, and differentiation of three B-cell- and one non-B-cell-derived human lymphoma cell lines were examined after treatment with protein kinase C-activating phorbol esters. Treatment with these agents resulted in early activation events similar to those observed in normal B cells. However, in contrast to their growth-promoting effect on normal human B lymphocytes, exposure to these phorbol esters induced profound growth inhibition of the three B-cell-derived lymphoma lines. Maximal inhibition was achieved within 24 hours of culture initiation and could be reversed if the phorbol ester was removed after 12, but not 20, hours in culture. Cell-cycle analysis of phorbol ester-treated lymphoma cells revealed a G1/S block in one line, whereas cells from the other two lines accumulated in G2/M. These data demonstrate that protein kinase C-binding phorbol esters can interrupt the cell cycle in two places in actively dividing human B-lymphoma cells. These findings may prove valuable with regard to potential therapy of human malignant lymphomas.