Newborn F344 rats were treated with polyclonal rabbit anti-thymocyte serum, which caused partial immunosuppression for a limited period. These rats were injected s.c. with cells from human tumor cell lines (two melanomas and a B lymphoma). Within 46 days after the tumor regression a renal carcinosarcoma, resembling Wilms' tumor, developed in each animal, however in male rats an adenocarcinoma of the preputial gland arised. Chromosome analysis and transplantation experiments proved the rat origin of the new tumors. In the present study we describe the biological and morphological characteristics of the adenocarcinoma of the preputial gland (ACPG). Their invasive properties were demonstrated in the lung colony assay, after intra-arterial injection and in the spleen-liver model. ACPG showed unique metastatic phenotype: dependence on the sex of the host.