Single-cell analysis by flow cytometry has enabled us to analyze the effects of a phorbol ester and known tumour promoter, TPA, on the phenotypes of four tumour lines. TPA is capable of triggering a variety of cellular alterations that can affect gene expression and the biochemical balance of intracellular events. We have investigated the effect of TPA on such properties as rate of proliferation, differentiation, expression of cell surface molecules, and susceptibility to natural killer (NK) cell-mediated cytolysis. Four human leukemia and lymphoma cell lines; K562, MOLT 4, Raji, and HL60, were studied in their response to TPA treatment. Based on measurements of the defined cellular properties, we have characterized the pleiotropic responses of each tumour cell line to the phorbol ester in relation to intensity and time of onset of each response. The effects of TPA are highly varied, ranging in time of onset from minutes to days, and in intensity from strong to weak within the four cell lines studied. However, within all the processes that are affected, the activation of protein kinase C appears to be a common initiating event of phorbol ester induction.