After kidney homotransplantation in dogs we pursued daily the macrophage migration inhibitory (MI) activity, which is one of the diagnostic parameters, and obtained the following results. At the time when it was diagnosed to have an acute rejection from routine clinical findings in the untreated control group, MI-activity was positive in 77% of the cases. The MI-activity was positive in 65% already 36 hours prior to the diagnosis was clinicaly made as of acute rejection case. With the test group treated with MPSS (methylprednisolone) MI-activity was positive in 75% of the cases at the time when diagnosed as of acute rejection. In 6 cases (43%) out of 14 the MI-activity was positive 48 hours prior to the diagnoses of acute rejection. When 20 mg/kg/day of MPSS was injected intravenously to 14 cases that had shown acute rejection despite Imuran administration for 3 consecutive days, serum creatinine decreased from 3.4 mg/dl to 2.4 mg/dl, urine excretion increased, and MI-activity also recovered to the normal range from 75% to 94% by a significant difference (P<0.001). These findings seem to indicate that the macrophage migration inhibitory activity test after kidney transplantation is not only an effective, immunological diagnosis to assess the acute rejection after kidney transplantation but also useful in determining the effect of immunosuppression.