Leukoregulin's effect on biochemical pathways involving Ca2+ was assessed in K562 erythroleukemia cells in which the antitumor lymphokine induces a rapidly reversible increase in plasma membrane permeability. Leukoregulin exposure activates protein kinase C but does not alter levels of phosphoinositol metabolites, calmodulin or cAMP. The activation pattern of protein kinase C differs from that induced by phorbol myristic acid (PMA), a potent activator of protein kinase C. PMA-induced translocation of protein kinase C from the cytosol to the plasma membrane is maximal by 2 min after addition of PMA whereas after leukoregulin treatment protein kinase C translocation reaches a maximum at 2 h. This suggests that leukoregulin activates protein kinase C via a non-classical phosphoinositol pathway as opposed to direct binding to protein kinase C as occurs with PMA. The temporal kinetics of the protein kinase C translocation and the increase in membrane permeability induced by leukoregulin are similar suggesting that phosphorylation of a membrane protein may be involved in the target cell destabilization of the plasma membrane by this lymphokine.