Sixty-six percent of rats immunized with laminin isolated from a mouse Engelbreth-Holm-Swarm (EHS) sarcoma developed moderate lesions in the testis characterized by multiple foci of seminiferous tubules with different degrees of sloughing of the germinal epithelium or atrophy intermingled with normal histological areas. Interstitial mononuclear cell infiltrates were seen in the epididymis. By electron microscopy, pathological changes in the basement membranes of the seminiferous tubules, such as splitting and focal thickenings of knob-like projections toward the epithelium, were observed. Moreover, Sertoli cell cytoplasm showed dilated smooth endoplasmic reticulum and large vacuoles. By electron microscopy with the immunoperoxidase technique, staining for in vivo-bound rat IgG was detected along the walls of the seminiferous tubules as a bright linear immunofluorescence and as a dense reaction product on the basal lamina. High titers of circulating antilaminin antibodies were detected by ELISA in all the rats immunized with laminin. As revealed by the skin test, a delayed type hypersensitivity reaction to laminin was observed in these rats.