Slye, in her 5th, 6th and 7th reports (1, 2, 3) on the inheritability of tumors in mice, cites 62 spontaneous tumors of the liver including adenomas, sarcomas, and carcinomas in a single charted family of related individuals. Outside of her extensive experience, a sporadic occurrence of such tumors is the rule. According to Slye, up to 1916 only one such tumor had been reported in the literature (4). Within the past three years, however, fourteen such tumors have occurred in mice of a single inbred strain.