A comprehensive study of the R-3327 line of prostate adenocarcinoma of the Copenhagen rat was performed. This tumor, investigated in its histologic, endocrinologic, and immunologic aspects, was compared with a squamous cell prostate carcinoma derived from the R-3327. The two tumor lines differ in their rates of growth and in their androgen receptor contents, i.e., the adenocarcinoma is androgen dependent and grows slowly, whereas the squamous cell carcinoma has a rapid rate of growth and is androgen independent. A study of cell-mediated immune responses revealed that: 1) Nonspecific responses to mitogens in the blastogenic assay, as well as antibody-dependent cellular cytotoxicity, are enhanced in animals bearing tumors. 2) The R-3327 is immunogenic to its host as demonstrated by two parameters. 3) The antigens present in the squamous cell carcinoma are not recognized by lymphocytes of the animals bearing the adenocarcinoma, substantiating the specificity of the reaction. For these reasons, the system of R-3327 prostate adenocarcinoma provides a relevant model for the study of prostate cancer.