Twenty-seven out of 31 cultured human cancer cell lines transplanted subcutaneously into nude mice produced solid tumors. Direct transplantation of surgical materials proved less successful. In 35 attempts, only 1 of 6 gastric cancers, 2 of 3 liposarcomas, and 1 of 3 osteosarcomas were accepted. No positive tumor formation was noted after the inoculation of 3 nasopharyngeal carcinomas or 1 breast cancer. None of lymphomatous neoplasms and leukemias produced any tumor in 12 and 3 attempts, respectively. The histological and cytological characteristics of the tumors developed were studied with light and electron microscopes, in relation to the features in vitro and those of the parent tumors. Histological similarity of the tumors that developed in nude mice to their parent tumors were noteworthy. Serial transplantation was performed successfully in 9 cell lines and 2 tissues. Preservation of tissue construction ability and other differentiation abilities of in vitro cultured human cancer cells were ascertained in varying degrees.