One mechanism by which drugs alter the function of enzymes is through chronic inhibition. To determine whether commonly used cancer chemotherapeutic agents could alter protein kinase C (PKC) and thereby modify the calcium-messenger system, we studied the effect of anthracyclines and vinca alkaloids on the activity of PKC. Doxorubicin, daunomycin, vincristine and vinblastine inhibited the activity of PKC by 50% at concentrations of 150, 120, 350 and 140 microM respectively. Furthermore, we demonstrated the potential for this interaction to occur in intact cells, since doxorubicin blocked the binding of the phorbol ester, PDBu, to its receptor, PKC. The mode of inhibition of PKC was due, at least in part, to interference with the activation of the enzyme by phosphatidylserine. The activity of PKC was increased 15 fold in a highly resistant human breast cancer line, but this increase in enzymic activity was not seen in all lines tested. These studies demonstrate that anthracyclines and vinca alkaloids inhibit PKC, and suggest that chronic antagonism could lead to changes in its activity and function.