In the initiation-promotion (I/P) assay for mouse skin tumorigenesis the initiating activity of various compounds is dependent upon both route of administration and strain of mouse tested. Urethane produces 3-fold more skin papillomas per mouse when administered orally than dermally in Sencar mice. To examine the biochemical basis for route and strain differences in the I/P assay, the binding of 14-C-urethane to DNA, RNA, and protein in mice susceptible (Sencar) and relatively resistant (Balb/c) to tumorigenesis by the protocol was determined. Binding of 14-C-urethane (0.062 mg/g body weight, 50 uCi/20 g body weight) to DNA, RNA and protein 6 hours after oral administration varied with tissue (liver > stomach > skin = lung) but not with strain. Binding to DNA in skin, lung and stomach, RNA in stomach and protein in stomach and liver after 48 hours was significantly higher in Sencar than in Balb/c. Dermal application of 14-C-urethane resulted in several fold higher binding to liver DNA of Sencar mice than Balb/c mice. Forty-eight hours after dermal application, significantly higher levels of 14-C-urethane remained bound to skin DNA, RNA and protein Balb/c mice, although all values were lower than at 6 hours after treatment.