Oleic, arachidonic, eicosapentaenoic and docosahexaenoic acids inhibited lymphocyte protein kinase C activity in the presence of Ca2+, phospholipid and a phorbol ester. Linoleic and alpha-linolenic acids did not affect protein kinase C activity in this way and none of the fatty acids affected protein kinase A activity. These findings indicate direct inhibitory effects of some unsaturated fatty acids upon protein kinase C. Culture of lymphocytes in the presence of oleic, arachidonic, eicosapentaenoic or docosahexaenoic acids resulted in a reduction in protein kinase C activity (by up to 45%). Culture with linoleic or alpha-linolenic acids did not affect protein kinase C activity and none of the fatty acids affected total protein kinase A activity or the percentage in the active form. These results show for the first time that fatty acids have long term effects upon protein kinase C activity, perhaps as a result of altering the rate of turnover of the enzyme. It is suggested that the inhibition of lymphocyte functions caused by unsaturated fatty acids may in part be due to their effect on protein kinase C.