We have determined the skin tumor-initiating activity in SENCAR mice of two A-ring derivatives of 7,12-dimethylbenz[a]anthracene (DMBA). 4-Fluoro-7,12-dimethylbenz[a]anthracene at a dose of 200 nmol per mouse exhibited weak activity, producing 0.6 papillopmas per mouse; doses of 10 and 20 nmol per mouse had no activity. A derivative of DMBA with the A-ring reduced, 1,2,3,4-tetrahydro-7,12-dimethylbenz[a]anthracene (1,2,3,4,-H-DMBA), had substantial tumor-initiating activity when compared with the parent hydrocarbon. In one experiment, doses of 10 and 100 nmol per mouse gave rise to 1.6 and 9.5 papillomas per mouse, respectively; similar results were obtained in 3 additional experiments. Although the tumor-initiating activity of 1,2,3,4,-H4-DMBA was approximately one-tenth that of DMBA, this derivative was slightly (17%) more active than benzo[a]pyrene. 1,2,3,4-H4-DMBA was tested for the ability to induce mutations to 6-thioguanine-resistance in Chinese hamster V79 cells. In the absence of feeder cells capable of metabolizing polycyclic hydrocarbons, it was not mutagenic. However, in a cell-mediated mutation assay with secondary hamster embryo cells as activators, this derivative produced mutations in a dose-dependent manner and was approximately one-tenth as active as DMBA. These results indicate that metabolism of DMBA at positions 1-, 3-, 2- and 4- is important for biological activity and that for certain derivatives (i.e., 1,2,3,4-H4-DMBA), alternate pathways of metabolic activation may also be important.