For several years a manufacturer of electric conduits had observed a persistent and troublesome dermatologic condition among his workers. The physicians previously consulted had been unsuccessful in solving the problem of its origin and in relieving the workers affected, and the management then requested an investigation by the United States Public Health Service. Schwartz, suspecting photosensitization dermatitis, asked the co-author, who had not been on the case previously, to participate in the investigation. After this an inspection was made of a similar manufacturing plant in another state and of related industrial plants elsewhere, and research as to the etiologic factors concerned was undertaken. It was observed by us that dermatitis and melanosis are not uncommon in industries where tar and pitch are handled, that the preparations are important occupational hazards and that this is not generally known in this country. A review of the literature indicated also that the mechanism