Prednisolone is oftenly used for the treatment of chronic active hepatitis (CAH), but it's therapeutic effect and mechanism of action are still unclear. Recently antibody dependent cellmediated cytotoxicity (ADCC) has been proved to be an important mechanism of liver cell injuries. This paper reports the effect of prednisolone on K-cell (the effector cell of ADCC) in vivo and in vitro experiments. K-cell assay was performed by plaque method using S-RBC as a target cell. Oral administration of prednisolone decreased the number of K-cell of peripheral blood lymphocytes in CAH (P<0.01) and clinical improvement was observed at the same time. In vitro study, prednisolone suppressed the human K-cell (P<0.05) at a low pharmacologic concentration such as 0.1 μg per ml. And the suppression of K-cell by prednisolone showed a dose response and was more prominent in male than in female. This suppression of K-cell by prednisolone was observed only when the lymphocytes were preincubated with prednisolone before mixing with anti S-RBC antibody. Washing the lymphocytes free of prednisolone reduced the suppression. On the contrary prednisolone did not suppress the K-cell when it was added to the lymphocytes after mixing with anti S-RBC antibody.