Immunological damage induced by iso-immunization of guinea-pigs with testis in adjuvant is most readily observed in the rete testis and vasa efferentia and is less frequently present in the testis and epididymis. Possible reasons for the greater vulnerability of the rete testis and vasa efferentia are discussed. The testis shows interstitial inflammation and an invasion of tubules by eosinophils and mononuclear cells. The tubular barrier to acriflavine, and possibly also to γ-globulin, partially breaks down during the invasive phase of initial damage but is restored before normal spermatogenesis is re-established. It is suggested that the occurrence of testicular damage is dependent upon the immune response, of whatever type(s), overcoming the protective effect of the blood-testis barrier.