Using 2 immunogenic (3-methylcholanthrene-induced fibrosarcomas in BALB/c x DBA/2 F1 (CD2F1) male mice, we observed initially that the rate of tumor growth might be enhanced by castration. For confirmation, tumor transplantation experiments with over 500 mice were done to compare tumor-specific transplantation immunity in castrate and in control male mice. Inbred mice bearing a 3-methylcholanthrene-induced fibrosarcoma transplant underwent surgical excision of the tumor; specific resistance to subsequent challenges with varying doses of that tumor cell line were compared in castrate and in noncastrate groups of mice. Although castration influenced the rate of tumor growth, it had no apparent effect on tumor-specific immunoresistance. Mechanisms of host-tumor immunorelationships are discussed as they might relate to endocrine therapy of prostate adenocarcinoma.