An acute and intense trauma was inflicted in one of the testes of adult guinea pigs. The immunological response and histological changes developed both in the injured and contralateral glands were studied at different intervals of time. It was observed: 1) Necrotic seminiferous tubules and a granulomatous cell reaction were seen in the injured gonad at 15 days. This was followed by a reduction of the mononuclear cell infiltration at 48 days. Finally a fibrotic tissue replaced the damaged areas at around 100 days. 2) Foci showing the classical picture of immunologic orchitis appeared in the contralateral gland at 15 and 48 days. Less frequent lesions were detected in the final period of the experiment. 3) These lesions were accompanied by cellular immunity detectable by the macrophage migration inhibition test at 15 and 48 days. With the exception of cytophilic activity also detected in same periods, no antibodies of hemagglutinating and sperm-agglutinating types could be found.