This report demonstrates for the first time that human peripheral blood neutrophils and human bone marrow cells metabolize arachidonic acid (AA) by a cytochrome P450 dependent mechanism. The formation of the cytochrome P450 (P450) arachidonate metabolites is dependent on the addition of NADPH, prevented by SKF-525A (100 microM), which is an inhibitor of cytochrome P450 enzymes, but not affected by the addition of BW-755C, a dual inhibitor of cyclooxygenase and lipoxygenase activities. Addition of the Ca+(+)-ionophore A23187 to cell preparations stimulated the release of both cyclooxygenase and lipoxygenase products but did not affect the formation of the P450 metabolites. Incubation of cell preparations with 14C-AA yielded a P450 dependent peak which eluted at 19 min. on reverse phase HPLC and was distinct from 5 and 15-hydroxyeicosatetraenoic acids (HETE's), prostaglandins and other leukotrienes. Recovery of the P450 dependent metabolite(s) was accomplished and preparations were tested in bone marrow clonal culture in order to determine if the substance(s) has/have any effect on the growth and differentiation of bone marrow cells. Results demonstrated that some of the material possessed powerful erythroid colony (CFU-E) enhancing activity at concentrations between 10(-8)-10(-14)M. These results demonstrate that human bone marrow and peripheral blood neutrophils possess a third pathway for AA metabolism which is P450 dependent, and that metabolite(s) of this pathway may have potent stimulatory effects on erythropoiesis.