In 70% of the patients with unilateral retentio testis the normal descended testicle shows damage in spermiogenesis too. The formation of auto-antibodies is considered as a possible cause. In rats with artificial uni- and bilateral cryptorchism, this mechanism was examined. The presence of auto-antibodies against spermatozoa could be proven by indirect immunofluorescence. After uni- and bilateral intra-abdominal displacement of the testicles, this phenomenon takes place after the 3rd month post-operatively, shows the maximum of titers after 6 months and decreases towards the 9th month post-operatively. An animal model is presented, suitable for the study of immunological processes in unilateral gonadal diseases.