The effects of UVB energy and nitrogen mustard (mechlorethamine, HN2) on tumor formation in the hairless mouse skin was examined. Twice weekly applications of HN2 produced tumors in 34% of the mice. In addition, the HN2 treatments plus UVB radiation resulted in a significant acceleration in tumor formation as compared to either carcinogenic stimulus alone. The twice weekly application of HN2 plus UVB exposure was more tumorigenic than UVB plus HN2 applied once a week. Thus a positive carcinogenic dose response was noted. No tumor-suppressing effects were detected in this study.