Using protein kinase C partially purified from mouse brain cytosol, we examined the effect of a number of phorbol ester and nonphorbol tumor promoters on protein kinase C enzymatic activity and [3H]phorbol 12,13-dibutyrate binding. Mezerein and phorbol 12-retinoate 13-acetate, second stage tumor promoters, as well as the weak tumor promoter 4-O-methylphorbol 12-myristate 13-acetate stimulated kinase activity to the same extent as did the complete tumor promoter phorbol 12-myristate 13-acetate. In contrast, the nonphorbol ester tumor promoters anthralin, cantharidin, benzoyl peroxide, and 7-bromomethyl-benz(a)anthracene did not affect kinase activity. The unsaturated fatty acids palmitoleic, oleic, linoleic, linolenic, and arachidonic acids, some of which have been reported to be weak tumor promoters, stimulated protein kinase C activity in the presence of phospholipids, as well as causing some activation in the absence of phospholipids. The saturated fatty acids butyric, lauric, myristic, and palmitic acids had relatively little effect. The fatty acids showed generally similar structure-activity relationships for inhibition of [20-3H]phorbol 12,13-dibutyrate binding as for stimulation of kinase activity. The unsaturated fatty acids typically decreased binding levels for the reconstituted aporeceptor, while the saturated fatty acids did not. The nature of this inhibition was explored in the case of arachidonic acid. Scatchard analysis demonstrated decreases in both the maximum number of binding sites as well as the apparent binding affinity, indicative of a complex mechanism. As expected for a lipophilic ligand, the effect of the arachidonic acid was reduced in the presence of elevated levels of phospholipid. Our results suggest that fatty acids are capable of modulating the phorbol 12,13-dibutyrate receptor:protein kinase C.