Corticosteroids have long been used for anti-inflammatory, anti-rheumatoid and other purposes in hospital. These effects may be due to inhibit immune reaction. So the animal given corticosteroids was more susceptible to infection because of immunosuppressive effect of corticosteroids. The purpose of this study was to investigate the effects of prednisolone on the lymphocyte subset in the spleen, immunoglobulin in serum, spleen weight, thymus weight and total WBC in peripheral blood. Mice were randomized into 3 groups. Each group has 24 mice. The small dosage group were given by 4 mg/kg/day of prednisolone for 4 days and the large dosage group were given by 8 mg/kg/day respectively. Prednisolone was suspended in saline and orally administered. Mice in control group were given saline alone. Eight mice in each group were sacrificed every week after administration of predisolone. The weight of thymus and spleen were mesured immediately. Lymphocytes were taken from spleen and these cells were analysed by flow cytometry. Also the concentration of total immunoglobulins in serum were assayed by enzyme-linked immunosorbant assay (ELISA). T cell, T-helper cell and T-cytotoxic cell were all significantly (P<0.05) decreased at 1 week after administration of predisolone and at 2 weeks they recovered similarly to that of control. Population of B cell showed various distribution. The concentration of total immunoglobulins in serum was not changed significantly. The weight ratio of spleen to body decreased significantly (P＜0.05) during predisolone administration but increased at 1 week later, Eventually the weight ratio was recovered to that of control at 2 weeks. The weight ratio of thymus to body decreased significantly (P<0.05) by prednisolone and recovered gradually up to normal ratio 2 weeks later,