In response to calcium ionophore (A23187) stimulation, human granulocyte/macrophage colony-stimulating factor-primed, dimethylsulfoxide-differentiated HL-60 cells (which resemble mature granulocytes) synthesized leukotrienes (LTs) LTA4, LTB4, LTC4, and LTD4. The synthesis of the sulfidopeptide LTs, LTC4 and LTD4, was specifically inhibited in cells incubated in the presence of both A23187 and phorbol-12-myristate-13-acetate (PMA), an activator of protein kinase C (PKC). In contrast, neither the synthesis of LTB4, a product of the nonpeptide branch of the LT pathway, nor the formation of LTA4, the precursor for both branches of the LT biosynthetic pathway, was significantly affected by the presence of PMA during A23187 stimulation. The inhibition by PMA of LTC4 production in A23187-stimulated HL-60 cells was dose dependent, with an IC50 value of approximately 3.5 nM. The PKC inhibitor staurosporine completely reversed the inhibition by PMA of LTC4 production in A23187-stimulated cells, in a dose-dependent fashion, with an IC50 value of approximately 30 nM. Bisindolylmaleimide, another PKC inhibitor, was also able to prevent PMA-mediated inhibition of LTC4 formation, whereas inhibitors of protein kinase A, tyrosine kinases, or the respiratory-burst oxidase were not. Measurement of LTC4 synthase enzymatic activity in cells challenged with A23187 and PMA in the presence or absence of staurosporine demonstrated that the activity of the LTC4 synthase enzyme was inhibited in cells costimulated with A23187 and PMA and that inhibition could also be completely prevented by the presence of staurosporine. Because PMA is known to activate PKC, and staurosporine and bisindolylmaleimide are inhibitors of PKC, these results suggest that LTC4 synthase in HL-60 cells may be phosphoregulated.