We studied the effect of fatty acids and their acyl-CoA esters on protein kinase C (PK-C) activity in human skin fibroblasts. Butyrate, octanoate, palmitate and oleate did not alter PK-C activity in either cytosolic or particulate fraction. In the presence of calcium, phosphatidylserine and diacylglycerol, both palmitoyl-CoA (Pal-CoA) and oleoyl-CoA (Ole-CoA) enhanced particulate PK-C activity by approx. 70% and octanoyl-CoA (Oct-CoA) by approx. 35%. Partially purified cytosolic PK-C activity was enhanced by 60–70% by 13.5 μM of either Pal-CoA or Ole-CoA. Basal histone phosphorylation (i.e., PK-C-independent phosphorylation) was decreased in the particulate fraction in the presence of these esters in a concentration-dependent manner. Both Pal-CoA and Ole-CoA fully substituted diacylglycerol in activating the kinase in both the cytosolic and particulate fractions, whereas Oct-CoA had a moderate effect. The pattern of endogenous cytosolic and particulate protein phosphorylation was altered in the presence of either Pal-CoA or Ole-CoA. We conclude that long-chain fatty acyl-CoA esters may activate PK-C in non-stimulated fibroblasts, i.e., in the absence of physiological diacylglycerol formation. Activation of PK-C in stimulated fibroblasts, i.e., in the presence of an elevated diacylglycerol concentration, is less pronounced. These results support the hypothesis that activation of PK-C and alteration of endogenous protein phosphorylation may play a role in the pathogenesis of diseases in which there is intracellular accumulation of fatty acyl-CoA esters, such as in inborn fatty-acid oxidation defects.