10-Fluoro-7,12-dimethylbenz(a)anthracene (10-F-DMBA) is a more potent skin tumor initiator in SENCAR mice when compared with the parent hydrocarbon 7,12-dimethylbenz(a)anthracene (DMBA). To elucidate the mechanism for this difference, the covalent binding of these two hydrocarbons to the DNA of mouse epidermal cells in vivo and in vitro was compared. The quantity of 10-F-DMBA covalently bound to mouse epidermal DNA in vivo was greater than that of DMBA at all doses tested over the range of 4 to 200 nmol/mouse. The magnitude of this binding difference between 10-F-DMBA and DMBA was greater at the higher doses (e.g., 1.5-fold at 4 nmol/mouse versus 3.4-fold at 200 nmol/mouse). These results correlated closely with the dose-response relationships for tumor initiation by the two hydrocarbons. Analysis of the isolated DNA samples by Servacel DHB chromatography revealed the relative proportion of syn-diol-epoxide:DNA adducts derived from DMBA increased dramatically as a function of dose (approximately 30% at 4 nmol/mouse versus approximately 55% at 200 nmol/mouse). Conversely, the relative proportion of syn-diol-epoxide adducts derived from 10-F-DMBA was low and remained essentially constant over the same dose range. High-pressure liquid chromatographic analyses of the DNA adducts derived from DMBA- and 10-F-DMBA-treated mice revealed qualitatively similar profiles. However, as expected, there was a marked reduction in the relative proportion of syn-diol-epoxide:DNA adducts in the profiles of epidermal samples from 10-F-DMBA-treated mice. The major syn-diol-epoxide:deoxy-adenosine adduct was present at a level only 30% that found in high-pressure liquid chromatographic profiles of DMBA samples. Similar results were obtained when primary cultures of mouse epidermal cells were treated with the hydrocarbons. The results suggest that the increased total binding and possibly the decreased proportion of syn-diol-epoxide:DNA adducts confer greater tumor-initiating potency on 10-F-DMBA.