The plasma myeloperoxidase (MPO) level was evaluated using a specific radio-immunoassay (RIA) for MPO in alpha 2b-interferon (IFN)-treated patients with chronic viral hepatitis. The plasma MPO was checked before and after the initial 2 weeks use of IFN at a dose of 6 x 10(6) U/day. The mean concentration of plasma MPO was found to be markedly higher after IFN therapy than that before the therapy (421.7 +/- 34.3 vs 242.9 +/- 23.0 ng/mL, P < 0.001). The plasma MPO negatively correlated with the granulocyte count (r = -0.37, P < 0.02) and the platelet count (r = 0.49, P < 0.01, while it positively correlated with serum alkaline phosphatase (ALP; r = 0.41, P < 0.03). The plasma MPO also showed a strong correlation with plasma polymorphonuclear granulocyte elastase (PMN elastase; r = 0.73, P < 0.001). Our study thus suggests that the increased release of MPO from destroyed granulocytes is responsible for the high concentrations of the plasma MPO in patients during IFN therapy, because the plasma MPO, PMN elastase and ALP abundant in granulocytes all increased in spite of a decrease in the granulocyte count. Granulocytopenia during IFN therapy may therefore be due to the increased destruction of granulocytes in addition to a direct suppression of the bone marrow by IFN.