SUMMARYAn attempt was made to inhibit the rejection of renal homotransplants by use of donor antigen, both in cellular and subcellular preparations. Kindeys were transplanted between unrelated dogs who were given antigen alone or antigen plus immunosuppresive drugs. Animals treated with Imuran alone were used as controls. In spite of a variety of different experimental protocols there was no evidence that antigen produces immunosuppression, on the contrary in some experiments there appeared to be sensitization. Possible reasons for the experimental results are discussed.