The requirement for protein kinase C (PKC)-beta in phorbol 12-myristate 13-acetate (PMA)-induced macrophage differentiation of human HL-60 promyelocytic leukemia cells was studied by using the variant HL-525, which is deficient in PKC-beta and is resistant to PMA-induced differentiation. Transfecting these resistant HL-525 cells with expression vectors containing either PKC-beta I or PKC-beta II cDNA resulted in clones that displayed PKC-beta transcript levels similar to or higher than those of the parental HL-60 cells or cells from a PMA-susceptible HL-60 clone, HL-205. These productive transfectants also exhibited PMA-induced cell attachment and spreading, inhibition of cell replication, reactivity to the OKM1 monoclonal antibody, and the ability to phagocytize opsonized beads, which are all characteristic macrophage markers. No PMA-induced differentiation markers were observed in any of the PKC-beta I or PKC-beta II transfectants that did not exhibit an increased PKC-beta RNA level or in cells transfected with control plasmids. These results indicate that restoration of the PKC-beta isozyme deficiency by productive gene transfection causes HL-525 cells to revert to a phenotype like that of the parental HL-60 cells, which is characterized by susceptibility to PMA-induced macrophage differentiation. Therefore, we can conclude that PKC-beta is one of the essential elements in the PMA-induced signal transduction pathway which leads to macrophage differentiation in HL-60 cells and perhaps in other related cell types.