Fatty acids can be substituted or phosphatidylserine in a reconstitution of phorbol ester binding to protein kinase C. 2. Phorbol ester, however, does not seem to be effectively utilized for the activation of the enzyme. 3. It is suggested that fatty acids play a role on the activation of protein kinase C in the abnormal conditions such as ischemia, while the phospholipid-dependent activation has a physiological significance in normal conditions.