Protein kinase C phosphorylated muscle and non-muscle monomeric actin more efficiently than filamentous actin in vitro. By sedimentation assay, the ratio of phosphorylated to unphosphorylated actin was much higher in sedimentable actin than in the non-sedimentable form, suggesting that phosphorylated actin was more readily incorporated into F-actin than unphosphorylated actin. In contrast, actin phosphorylated by cAMP-dependent protein kinase was found to have weaker polymerizability than the unphosphorylated form. The phosphopeptide mapping pattern of actin phosphorylated by protein kinase C was different from that of actin phosphorylated by cAMP-dependent protein kinase. Thus, both the protein kinases phosphorylate actin differently and induce opposite effects on actin polymerizability.