Protein kinase C has been shown to be a phospholipid/Ca2+-dependent enzyme activated by diacylglycerol (Nishizuka, Y. (1984) Nature 308, 693-697; Nishizuka, Y. (1984) Science 225, 1365-1370). We have reported that unsaturated fatty acids (oleic acid and arachidonic acid) can activate protein kinase C independently of Ca2+ and phospholipid (Murakami, K., and Routtenberg, A. (1985) FEBS Lett. 192, 189-193). This study shows that other cis-fatty acids such as linoleic acid also fully activate protein kinase C in the same manner. None of the saturated fatty acids (C:4 to C:18) nor the detergents (sodium dodecyl sulfate and Triton X-100) tested here were as effective as oleic acid. Unlike oleic acid, these detergents strongly inhibited protein kinase C activity induced by Ca2+/phosphatidylserine (PS) and diacylglycerol. Lowering the critical micelle concentration of oleic acid by increasing ionic strength also strongly inhibited oleic acid activation of protein kinase C activity. Dioleoylphosphatidylserine activated protein kinase C effectively (Ka = 7.2 microM). On the other hand, dimyristoylphosphatidylserine, which contains saturated fatty acids at both acyl positions, failed to activate protein kinase C even in the presence of Ca2+. These observations suggest that: protein kinase C activation by free fatty acid is specific to the cis-form and is not due to their detergent-like action, cis-fatty acid activation is due to the direct interaction of protein kinase C with the monomeric form of cis-fatty acids and not with the micelles of fatty acids, and cis-fatty acids at acyl positions in PS are also important for Ca2+/PS activation of protein kinase C.