Between July, 1981 and June, 1984, 40 malignant urogenital neoplasms were heterotransplanted into nude mice to clarify the biological characteristics of malignancies and screen the effective anticancer drugs against them. The primary tumor take was confirmed in 7 tumors (17.5%), including 4 of 14 renal cell carcinomas (28.6%) and 3 of 13 bladder cancers (23.1%). No primary take was recognized in renal pelvic, ureteral, prostatic and testicular cancers. Three of the 7 tumors (43%) with primary take (7.5% in total) have been serially transplanted for more than a year. These three established tumors were all renal cell carcinomas and were designated as AM-RC-1, AM-RC-6 or AM-RC-11. The take rate of tumor tissues obtained from primary and metastatic lesions was 18.2% (6/33) and 14.3% (1/7), respectively. The duration from tumor inoculation to take was examined in seven transplantable tumors. The shortest was 12 days in renal cell carcinoma, the longest was 36 days in bladder cancer and the average was 24.4 days. The correlation between the time from tumor extraction to inoculation and transplantability was investigated. The take rate was 29.4% within 3 hours, and 18.2% beyond 3 hours. The obvious effect of anti-asialo GM1 antibody on tumor take, which inhibits the natural killer activity of mice or rats selectively, was not demonstrated owing to the small number of cases. The histopathological features of seven original tumors with primary take were examined. All of the 4 renal cell carcinomas were shown to be histologically high grade and advanced. Two of the 3 bladder cancers were poorly differentiated. Therefore, the differentiation or growth rate of the tumor was suggested to be closely correlated with transplantability. The three strains of renal cell carcinomas transplanted serially for a long-term period, AM-RC-1, AM-RC-6 and AM-RC-11 disclosed a constant growth after the 7th, 9th and 10th inoculation respectively. The renal cell carcinomas serially transplantable to nude mice maintained the basic histologic findings of the original tumor even after long-term serial transplantation.