Dermal melanocytosis is a natural state for many mammals. In the human primate, it appears only as a congenital or atavistic remnant in nevus fuscocaeruleus ophthalmomaxillaris of Ota, the mongolian spot, and in the blue nevus. Its development has been thought due to arrested migration of pigment cells, and this condition has not been regularly produced in human tissue by any means. Szabo1 has induced a similar condition in the mouse by painting with methylcholanthrene and croton oil. Rappaport and associates2 have induced an unusual tumor that they term a cellular blue nevus in Syrian hamsters by means of dibenzathracene. This is a report of our experience in the induction of the blue nevus in the skin of the hairless mouse. Method Six hairless mice were painted with gas-oil, a low-viscosity heating fuel, and exposed to ultraviolet irradiation daily. The ultraviolet source was the H85-C3 General Electric bulb,