A study was done to demonstrate quantitatively and graphically the way in which suberythemogenic doses of broadband UVA and UVB interact in producing a visible erythema. On the backs of fair-skinned human volunteers the minimal erythema dose (MED) was determined for polychromatic UVA and UVB. Increasing fractions of the UVA MED were given to sites already exposed to various fractions of the UVB MED resulting in sites exposed to various doses of both UVA and UVB. The same experiment was repeated with the order of wavebands reversed. It was demonstrated that when UVA was followed by UVB an erythema was produced in those sites where the sum of the fractions was equal to one, an interaction termed photoaddition. When the UVA exposure followed the UVB, erythema was again predominantly noted in those sites demonstrating photoaddition. However, in the latter case, numerous sites of threshold erythema were noted where the sum of the fractions was greater than one. This is suggestive of photorecovery. No evidence of photoaugmentation was observed with either order of exposure.