The role of UV radiation in the development of malignant melanoma has yet to be clearly defined. The purpose of these studies was to determine whether UV irradiation of mice produces local or systemic alterations that increase the in vivo growth of transplanted melanoma cells. K-1735 melanoma cells were injected into the external ears of syngeneic C3H mice. UV irradiation of the mice before or at the time of injection of the melanoma cells accelerated the appearance of the tumors. The effect was observed when melanoma cells were transplanted directly into the site of UV irradiation, but not when they were injected into an unirradiated site. The initial survival of radiolabeled melanoma cells at the site of inoculation was not altered by UV irradiation of the host, suggesting that the accelerated appearance of tumors was due to an increase in the clonogenic potential of cells injected into UV-irradiated skin. The effect of UV irradiation on the development of other syngeneic tumors was also investigated. The outgrowth of a second melanoma was also accelerated in UV-irradiated mice, whereas the growth of a UV-induced fibrosarcoma, a methylcholanthrene-induced fibrosarcoma, and a spontaneous hepatocarcinoma was not affected. These results suggest that, in addition to its carcinogenic activity, UV radiation may contribute to the incidence of cutaneous melanoma because of a local effect on the skin that stimulates melanoma development.