Pretreatment of guinea pigs with homologous testicular antigen (TA) in incomplete Freund's adjuvant (IFA) markedly reduced both the incidence and severity of experimental allergic orchitis (EAO) when animals were subsequently challenged with TA in complete Freund's adjuvant (CFA) and examined at 2 weeks after challenge. The pretreatment rendered lymph node lymphocytes, particularly T cells, specifically unresponsive to the subsequent orchitogenic challenge as judged by antigen-induced in vitro lymphocyte proliferative response and blastogenic factor production, whereas antisperm antibody formation was not affected. The suppression induced was of a transitory nature, lasting 3 weeks after orchitogenic challenge, but between 4 and 5 weeks after challenge the development of EAO and aspermatogenesis became evident in parallel with the restoration of cellular immune responsiveness. Cyclophosphamide treatment 3 days before orchitogenic challenge abolished the preventive effect of pretreatment with TA in IFA. Transfer of serum obtained from pretreated animals failed to prevent actively sensitized recipients from developing EAO, which does not favor a possible occurrence of serum blocking factors in protected animals.