In previous papers, we have reported that methylguanidine (MG), a known uremic toxin, was synthesized from creatinine (Cr) by active oxygen generated not only by chemical reagents but also by isolated rat hepatocytes. In this paper, we studied whether or not active oxygen generated by stimulated human neutrophils produces MG from Cr. MG was measured after incubating 2 x 10(6) human neutrophils for 2 h in 1 ml of Hanks' balanced salt solution (pH 7.4) containing 100 mg/dl Cr at 37 degrees C after the addition of phorbol myristate acetate (PMA). MG was measured by high pressure liquid chromatography followed by reaction with 9, 10-phenanthrenequinone. MG was synthesized by the stimulated neutrophils and not by the unstimulated ones. MG synthesis reached a plateau (1.11 +/- 0.03 nmol/120 min/2 x 10(6) cells) at a concentration of 0.125 microM PMA and reached a maximum value (1.95 +/- 0.03 nmol/120 min/2 x 10(6) cells) at a concentration of 100 mg/dl Cr. MG synthesis increased depending on the concentration of neutrophils between 1 and 8 x 10(6)/ml and increased depending on the duration of incubation up to 4 h. MG synthesis was strongly inhibited by superoxide dismutase, by the scavengers of hypochloride (taurine and methionine) and by sodium azide. Catalase and the scavenger of the hydroxyl radical (dimethyl sulfoxide) inhibited MG synthesis less effectively. The effects of the scavengers of active oxygen suggest the participation of active oxygen in MG synthesis from Cr in this system. Among the active oxygen species, superoxide anion and hypochloride play an important role in this system.(ABSTRACT TRUNCATED AT 250 WORDS)