Subjects experimentally photocontact sensitized to tetrachlorosalicylanilide (TCSA), tribromosalicylanilide (TBS), and bithionol were studied with regard to the quantity of long ultraviolet rays (UV) necessary to elicit reactions. Less than one second of high energy long UV, about 20 seconds or less of midday summer sunlight, will elicit photoallergic reactions in highly sensitized persons. Unlike sunburn, photoallergic reactions can readily occur on the hairy scalp and palms, though usually of lesser intensity. Reactions can be elicited through cotton and woolen cloth, adhesive tape, and bond paper. Two thicknesses of desk blotter paper afford a complete light seal.