A rapid screen assay for protein kinase C was described which allowed the detection of inhibitors and activators of the enzyme at different states of activation. Upon secondary evaluation of the active inhibitors, three classes of compounds were identified. The inhibition of protein kinase C by one class of compounds, exemplified by trifluoperazine, could be reversed in the presence of excess phosphatidylserine, indicating the phospholipid-interfering nature of these compounds. While the other two classes of compounds, represented by apomorphine and LY 170198, respectively, did not exert their inhibition by interfering with phospholipids, their inhibitory potencies differed depending on the state of activation of protein kinase C. LY 170198 was equally potent in the inhibition of protein kinase C either in its partially activated state in the presence of low concentrations of phosphatidylserine or in the diolein-stimulated state. Apomorphine, on the other hand, was less active against protein kinase C in the partially activated state. Isoquinolinesulfonamides have the same properties as those of the apomorphine and have been shown to compete for ATP binding. The mechanism of inhibition of protein kinase C by LY 170198 needs to be further investigated.