Calcium/phospholipid-dependent protein kinase (PKC) was partially purified from P1798 lymphosarcoma. Phospholipid-dependence was specific for phosphatidylserine. PKC phosphorylated Histone 1, with an apparent K_m of 14.1 μM. Chlorpromazine, a lipid-binding drug, inhibited PKC activity by 100%. Further studies were undertaken to establish analytical conditions which could be applied to the study of PKC in intact cells. The conditions included (1) determining optimum cell concentration for measuring PKC activity, (2) recovering PKC into the soluble fraction of cell extracts, (3) evaluating calcium and phospholipid requirements of PKC in this fraction, and (4) inhibiting PKC in this fraction. Final studies involved treatment of intact cells with potential activators. Both phytohaemagglutinin and a phorbol ester increased PKC activation.