The B oligomer of pertussis toxin serves as a weak mitogen in the T lymphocyte, an effect which is associated with an early rise in cytosolic free calcium concentrations, as monitored by Fura-2 fluorescence. Upon co-administration of phorbol dibutyrate, a phorbol ester tumour promotor which activates protein kinase C, pertussis toxin-induced proliferation was synergistically enhanced, as measured by the increased uptake of [3H]thymidine, into cellular DNA. Although phorbol ester co-administration has often been associated with an inhibition of Ca2+-mobilizing pathways, phorbol dibutyrate pretreatment had no inhibitory effect on the pertussis toxin-induced calcium flux and may actually have enhanced this response slightly. Flow cytometric analysis of cell populations expanded by the combined regimen did not provide evidence for the preferential expansion of cells bearing either CD4 or CD8, the T-cell determinants representative of the helper-inducer and cytotoxic-suppressor subsets, respectively. Pertussis toxin and phorbol dibutyrate appear, therefore, to elicit polyclonal stimulation, rather than the selective activation of a given lymphocyte subset. Expression of the transferrin receptor, a marker for nutrient uptake, and CD25, the Tac component of the interleukin-2 (IL-2) receptor, was, however, synergistically enhanced in cells activated by the co-treatment procedure.