Polymorphonuclear (PMN) phagocytosis was tested in three groups of 12 patients (non drug-addicts) hospitalized for acute A, B and non-A, non-B hepatitis. The same test was performed in 12 parenteral drug-addicts (P.D.A.) with non-A, non-B hepatitis, in 12 P.D.A. with type B hepatitis and in 30 P.D.A. without evidence of acute hepatitis. Percent of phagocytosis was evaluated at time of admission to hospital, at discharge and three months after the acute episode. A statistically significant reduction of phagocytosis was present in each of the non-addicts group at time of admission to hospital. At the discharge this reduction was confirmed for type B and non-A, non-B hepatitis, whereas an improvement was noted for type A hepatitis. Control tests carried out 3 months later evidenced that also in patients affected with type B and non-A, non-B hepatitis the percentages of phagocytosis had reached almost normal values. An increased percent of phagocytosis resulted from incubation of patients' PMN with normal human serum. Among drug-addicts the percentages of phagocytosis do not differ from the data recorded in non-addicts patients during the acute stage of the disease. However, three months after the acute episode, a deficit of phagocytic activity was still present in drug-addicts. These data would support the hypothesis that a serum factor impairing the PMN phagocytosis in acute hepatitis is present. Circulating immune complexes were found in the same sera. They might play an important role in inhibitory effect on neutrophils phagocytic activity.