Cell suspensions of tumor fragments derived from spontaneous or chemically induced primary hepatocellular carcinomas obtained from inbred C3H/HeN mice were transplanted into young male mice of the same strain. Transplantable hepatocellular carcinomas were excised as soon as they were detected, and all recipient mice were killed at one year of age. In control C3H/HeN mice, the incidence of primary hepatocellular carcinomas was 41% (41 of 100). In mice in which there was no growth of transplantable carcinomas, whether originally given injections of tumor cell suspensions or fragments, the overall incidence of primary hepatocellular carcinomas was 49% (35 of 72) with one to six tumors per liver at time of sacrifice. Transplantable hepatocellular carcinomas were established only in mice that had received tumor fragments. In these mice, from which established transplantable hepatocellular carcinomas had been excised, the overall incidence of primary hepatocellular carcinomas was 12% (3 of 25) with one tumor being found in each of three livers. The time of appearance or excision of transplantable hepatocellular carcinoma did not affect this decrease in primary hepatocellular carcinoma incidence.