Thirty-three male BB/W rats with diabetes of 11-145 days duration were divided into 3 groups: six received abdominal, intratesticular islet allografts and no immunosuppression posttransplantation; 15 were similarly grafted and in addition were given four injections of ALS regularly at 10-day intervals for 30 days after transplantation; and 12 rats received scrotal, intratesticular islet allografts and four injections of ALS. RESULTS in the absence of immunosuppression all six of the rats with abdominal, intratesticular islet allografts became normoglycemic within 2 days after transplantation but in none did graft survival exceed 17 days; a marked prolongation of graft survival of greater than 65-441 days occurred in 13 of the 15 animals with identical intratesticular allografts; sustained immunosuppression was not needed for prolonged islet allograft survival in rats with cryptorchid islet allografts; and (4) only one of the 12 rats with scrotal, intratesticular allografts became normoglycemic whereas 11 remained severely glycosuric. However, on surgical translocation of the grafted testes from the scrotum into the abdominal cavity, the rats promptly became normoglycemic in the absence of any additional therapy.