128 tumors from three different human renal-cell carcinomas and from one human transitional bladder cancer were transplanted into NMRI mice treated with cyclosporin A at various dosages. Their acceptance rate, proliferation rate, and morphologic and growth characteristics were studied and compared with those of the same human tumors after transplantation into NMRI nu/nu mice. The acceptance rate was 80% in mice treated with cyclosporin A at 100 mg/kg-1 and 150 mg/kg-1/day. The highest acceptance rate was observed after transplantation into nu/nu (thymus-aplastic) mice (100%). Morphologically, the transplanted tumor was similar in the two animal groups and identical with the primary tumors. Metastases were not seen in either animal model. The NMRI mice treated with cyclosporin A had leukocyte infiltration into the transplanted tumor. The growth rate of the human tumors in normal mice depended on the cyclosporin A dosage, and was highest in the nu/nu mice. The proliferation rates of the transplanted human tumors, as judged by flow cytometry, were rather similar in all groups.