The presence of an amelanotic form of melanocyte located in the middle portion of the outer root sheath of the human hair follicle has previously been described and discussed by one of us (1, 2, 3). Activation of" precursor melanocytes" in the hair of rabbits by the local application of methyl-cholanthrene in benzene was obtained by Quevedo and Isherwood who indicated the irritating chemical stimulus as the possible cause of such activation (4). It seemed possible that other stimuli would be capable of such activation. On the basis of such premises we exposed a well-defined hairy area of the chest of a 25 year old, fair-complected white male, to a daily erythem-atous dose of ultraviolet rays (Hanovia Alpine) in a gradually increasing amount for ten days, taking a biopsy specimen of the treated area and of an adjacent unexposed area on the 11th day. Skin-split preparates were obtained from both biopsy specimens by the sodium bromide method and were thereafter treated with dopa (5). Observation: In the irradiated area, dopa-