Spontaneous Tumors in ClH-Avy and ClH-AvYfB Mice: High Incidence in

SUMMARY-C3H-AvY an,d C3H-AvYFB mice, with a virtual 100 % incidence of liver and mammary tumors in the United States, had an almost 00/0 incidence of these tumors when these mice were bred and reared in Australia, particularly after the first generation. C3H-AvY mice, when reared in Australia but provided with U.S. feed and bedding (cedar shavings), also had essentially a 100 % incidence of mammary tumors. Although our results did not rule out other possibilities, the implication of cedar as a "carcinogenicU agent was quite clear-cut. These observations, although not yet made on large numbers of mice, provide new parameters For the study and understanding of the development of spontaneous mammary and liver cancer in mice.-J Natl Cancer Inst 50: 1237-1242, 1973. THE OCCURRENCE of spontaneous neoplasms is high in inbred strains of mice (1). In some strains the incidence may approach 100 % for tumors of a particular tissue. Heston and Vlahakis (2) and Vlahakis et al. (3) reported that C3H-Avy and C3H-AVYfB mice have a 90-100 % incidence of both liver tumors and mammary tumors