Synopsis Photocontact allergy, an acquired altered reactivity of the skin to light in the presence of a photosensitizer, has for many years been considered to be a delayed-type hypersensitivity. The response has been postulated as being mediated via the formation of a protein-photoallergen conjugate acting as a complete antigen. The purpose of this paper is to bring together evidence at the molecular level which supports this theory of photoallergy. All photoallergens studied so far have been shown to be able to bind to proteins under the influence of ultraviolet light. Photoallergen-protein binding in most cases is non-specific; the exception, that of tetrachlorosalicylanilide (T(4)CS), displays a high specificity towards serum albumin. The mechanism of protein-photoallergen binding is thought to proceed via the formation of highly reactive species such as free radicals. Free radicals have now been observed using electron spin resonance spectroscopy for at least five photoallergens. Macrophage inhibition and lymphocyte transformation experiments have indicated that protein-photoallergen conjugates act as complete antigens. Further evidence for this is provided by the observation that photoconjugates injected into guinea-pigs can induce a photoallergic response in the absence of irradiation. The response produced by T(4)CS-serum albumin conjugates is greater than that produced by any other combination of photoallergen and protein. The potency of the T(4)CS-serum albumin photoconjugate in inducing photoallergy, together with the binding specificity of T(4)CS, suggest that albumin may have a special role as a carrier protein in T(4)CS photoallergy.