Of 24 patients with yellow tattoos, 18 observed a swelling reaction in them when they were exposed to sunlight. Four of these patients also observed a similar type of reaction in the red tattoos. Tattoos done experimentally with cadmium sulfide were shown to give a swelling reaction when exposed to the wavelengths 3,800, 4,000, and 4,500 Angstroms (A). Photopatch testing with cadmium sulfide was negative. Cadmium sulfide is a compound with marked photoconducting properties. The swelling reaction provoked by light in tattoos of cadmium sulfide seems to be phototoxic in nature.