A specific fluorescence is developed in melanocytes, nevus cells, and cells of malignant melanoma by treatment of the tissue with dry formaldehyde gas. The fluorescence is often stronger in melanocytes adjacent to nevi or melanomas than in normal melanocytes. The strongest fluorescence occurs in cells of malignant melanoma. Among the limited number of compounds that condense with formaldehyde to fluorescent derivatives, DOPA [beta(3,4-dihydroxyphenyl)L-alanine] seems the most likely substance to give rise to the fluorescence observed in these lesions.