Photochemical and immunologic knowledge about photoallergy to chemicals is briefly summarized. Studies in in vitro systems have demonstrated that photoallergic compounds can covalently bond to proteins through a photochemical reaction. The immunologic nature of the photoallergic response is based mainly on clinical observations, induction of photoallergy in man and in guinea-pigs and on results of in vitro immunologic tests. Studies of the photoreactions of the photoallergic compound, 3,3',4',5-tetrachlorosalicylanilide (TCSA) with proteins are discussed. TSCA noncovalently bonds to human serum albumin prior to irradiation. Prior interaction is essential for formation of a photoaddition product indicating that a short-lived reactive species derived from TCSA is involved in the photoaddition and limiting the number of skin proteins which can participate in antigen formation. By fragmentation of the TCSA-albumin photoadduct with CNBr, it was determined that TCSA can bond to at least three sites on the albumin molecule. TCSA alone can sensitize the photooxidation of histidine in albumin.