The influence of horse antihuman thymocyte globulin (ATG) on renal allograft survival was assayed between March 1977 and August 1978. Fifty consecutive patients were randomly assigned to the treatment or the control group. Patients in both groups received azathioprine and prednisolone. ATG was added in the experimental group. The daily dosage was such as to maintain the rosette-forming cell (RFC) level at 10% of baseline values. The number of renal failure episodes (RFE's)was significantly lower in the experimental group during the period (1 month) ATG was given than it was in the control group during that same period of time. Two years after transplantation, kidney survival was 79% in the ATG group and 52% in the control group. The amount of steroids necessary to control RFE's was significantly lower in the ATG group. The dosages of ATG used in the experimental group were relatively small (average daily dose, 3.08 +/- 0.26 mg/kg of body wt), and adverse reactions were mild. Blood tolerance was excellent. T-cell monitoring thus appears to be an effective method in evaluating the daily dosage of ATG.