We identified aberrations leading to spontaneous autoimmune orchitis (AIO) in mink, a seasonal breeder and natural model for autoimmunity. This study provides evidence favouring the view that a malfunction of the clearance mechanisms for apoptotic cell debris arising from imbalances in phagocyte receptors or cytokines acting on Sertoli cells constitutes a major factor leading to breakdown of self-tolerance during spontaneous AIO. Serum anti-sperm antibody titers measured by ELISA reflected spermatogenic activity without causing immune inflammatory responses. Orchitic mink showed excess antibody production accompanied by spermatogenic arrest, testicular leukocyte infiltration and infertility. AIO serum labelled the post-acrosomal region, the mid and end piece of mink sperm whereas normal mink serum did not. Normal serum labelled plasma membranes whereas AIO serum reacted with germ cell nuclei. Western blot analyses revealed that AIO serum reacted specifically to a 23 and 50kDa protein. The number of Apostain-labelled apoptotic cells was significantly higher in orchitic compared to normal tubules. However, apoptosis levels measured by ELISA in seminiferous tubular fractions (STf) were not significantly different in normal and orchitic tubules. The levels of CD36, TNF-, TNF- RI, IL-6 and Fas but not Fas ligand (L) and ATP-binding cassette transporter ABCA1 were changed in AIO STf. TNF and IL-6 serum levels were increased during AIO. Fas localized to germ cells, Sertoli cells and the lamina propria of the tubules and Fas-L to germ cells. Fas colocalized with Fas-L in residual bodies in normal testis and in giant cells and infiltrating leukocytes in orchitic tubules.