The mitogenicity of 12-O-tetradecanoyl phorbol-13-acetate (TPA) for normal human peripheral blood mononuclear cells was investigated. TPA was a weak mitogen giving simulation indices in the range 2.5-10.5 at the optimum concentration (10 ng/ml) compared with 39-95 for phytohemagglutinin (PHA) at its optimum concentration (1 .mu.g/ml). No absolute requirement for a comitogen could be demonstrated, however TPA and PHA were syngergistic in their action at low concentrations, and additive at optimum concentrations. Cell fractionation by rosetting with sheep erythrocytes showed that most of the proliferative response to TPA occurred in the T cell fraction, however, some proliferation of non-T cells was also observed. Surface marker studies showed that this could not have been due to residual T cells in the non-T fraction. A small number of monocytes was required for optimal proliferation of T cells in response to TPA. After a 3 day incubation with mitogen, the responding cell populations were tested for binding of a range of antibodies spcific for T cell (OKT3, OKT4, OKT8 and OKT11), natural killer (NK) cell (anti-Leu-7), monocyte (FMC17) and B cell (anti-human Ig) surface markers. The responding cell types were T cells and B cells, but not NK cells or monocytes. Marker modulation of the antigen detected by OKT4, and to a lesser extent that detected by OKT3, in the presence of TPA, precluded determination of which subpopulations of T cells proliferated in response to TPA. TPA was also tested for its ability to maintain activated T cell blasts in a standard assay for interleukin 2 (IL-2). Mitogen-activated T cells were strongly responsive to TPA in this assay, but progressively lost responsiveness when maintained in crude IL-2 for .apprx. 2 wk. TPA does not have maintenance (i.e., IL-2-like) activity. Small amounts of TPA acted synergistically with PHA in maintaining blast populations which were not responsive to TPA alone. This illustrates the importance of using long-term IL-2-dependent cell lines for quantitation of IL-2 in supernatants prepared by stimulating T cells with these agents.