Between November 24, 1962, and May 15, 1963, 12 renal homografts were done at the University of Colorado Medical Center. In half of the cases, a kidney was provided by a donor of the same major blood type as the recipient. In the other half, the major blood groups of the donor and recipient patients were different. The present study is concerned with an analysis of the results in these comparative series, in order to determine what influence the presence or absence of major blood group compatibility had upon the early success rate.