The testis is an immunologically privileged site. Since earlier studies excluded testicular steroid production as an essential factor, the present study evaluates the role of germ cells and spermatogenesis in the privileged survival of allografts within the testis. We used a Sertoli cell-only testis model and adolescent unilateral cryptorchidism in inbred rats to eliminate germ cells and spermatogenesis selectively. Parathyroid allografts were implanted into these sites, normal testes and beneath the renal capsule (a nonprivileged site) in appropriately matched controls. With at least 15 rats in each group, privileged allograft survival was shown to be unaffected by eliminating germ cells and spermatogenesis (p less than .005). Experimental evidence suggests the presence of an active process which incidentally permits privileged allograft survival within the testis, but which exists teleologically to protect the developing sperm from autoimmune attack. This is in addition to the passive anatomical separation provided by the blood-testis barrier. Our cumulative data strongly implicates the Sertoli cell in this process.