Exposure of various cell types (rat-1 fibroblasts, bovine adrenocortical cells, human lymphoid cells) to nanomolar concentrations of TPA, resulted in a rapid, apparent loss of cellular protein kinase C content, when the enzyme was assayed by its phospholipid and Ca2+-dependent histone (H1)-kinase activity, following solubilization and DEAE-cellulose chromatography isolation. By contrast, no loss of protein kinase C was detected when the enzyme was probed by its high affinity PDBu binding capacity nor when the kinase activity was assayed with protein substrates other than histones, such as vinculin and a cytochrome P-450. It is concluded that, in addition to the previously reported enzyme subcellular redistribution, following TPA treatment, the phorbol ester induces striking alterations of the cellular protein kinase C catalytic activities. The molecular mechanisms of these changes and their implication in the tumor promotion process remain to be clarified.