After complete ischaemia of mouse, rat and rabbit left testes produced by ligating the funiculus spermaticus, a diminution and degenerative changes occurred in the testes following a temporary swelling. Degenerative changes were associated, in addition to necrosis of all cells in the testes, with an infiltration of leucocytes, mainly polymorphonuclear and mononuclear cells, and lymphocytes from the neighbourhood of the testis through the tunica albuginea into the surface parts of testicular tissue. The cells enumerated above migrated gradually from the periphery to the centre of the testis. First, they colonized the interstitial tissue and then perturbed the membrana propria of the seminiferous tubules and got inside the tubules. In the majority of animals, the appearance of connective tissue with collagen fibres was noted behind the zone of leucocytes and lymphocytes while the testis was shrinking. Ischaemic testes disappeared completely within 6 to 10 weeks after devitalization in most of the mice. Antibody production in the course of the degenerative process in the ischaemic testes was not demonstrated by indirect immunofluorescence nor was it detectable by spermagglutination test in rabbits, and the skin tests in rats and rabbits were also negative. The contralateral, non-ischaemic testes were not damaged in any of the examined males as shown by histological analysis. A similar degenerative process occurred in most of the testes of mice in which syngeneic SAI sarcoma cells grew and were implanted intracutaneously on the back of mice of the same inbred strain. Upon intraperitoneal inoculation of tumour cells the growth of ascites tumour appeared 5 to 8 weeks after implantation of the tumorous testis in about 30% of the mice, 70% of mice were resistant. Ascites tumours were demonstrated in control animals without previous tumour implantation and deaths were recorded in 95% of mice.