Suckling 86AF1/J hybrid mice were treated once by stomach tube with urethan at 1 week of age. Some of these hybrids received subsequent repeated treatment beginning at 4 weeks; others were treated once only with the same carcinogen on a body-weight basis and on day 1 to day 28 postpartum. Treatment of 1-week-old hybrids once with urethan resulted in the development of hepatomas in 33 percent of the males and in none of the females. When the same treatment was followed 3 weeks later by 15 additional treatments with urethan, large numbers of pulmonary adenomas developed. However, the incidence of hepatomas remained unchanged in both sexes. The liver showed a greater tumorigenic response to urethan than any other organ except the lung. Both age of suckling mouse at time of treatment with urethan and sex inAuenced liver tumorigenesis. Tumor yield was consistently greater for the males than for the females in each age group. Based on tumor yield, a peak response was observed for mice of both sexes treated on day 7. Also, the yield of hepatomas decreased more slowly for males than for females after day 7 and reached control levels on day 28 for the males and on day 21 for the females. Lung tumorigenesis, in contrast to liver tumorigenesis, was not influenced by age of suckling mouse at time of treatment and sex. Harderian gland adenomas and forestomach papillomas also were observed among experimental mice, the data suggesting that their induction may have been enhanced by treatment with urethan. The utilization of immature mice in bioassay testing procedures for potential carcinogens is discussed.