A single UVB or PUVA exposure given 4 days prior to skin testing affected skin responses both to contact allergens and to histamine and the histamine liberator, compound 48/80. The delayed contact hypersensitivity reactions were attenuated by UVB in 75% and by PUVA in 79% of the tests. The immediate skin reactions to histamine and compound 48/80 were diminished by UVB in 81% and by PUVA in 46% of the cases. While the epidermal Langerhans cell (LC) density was distinctly affected by irradiation, the attenuation of skin hypersensitivity reactions seemed to be independent of the degree of LC depletion. A significant correlation was, however, found between the strength of the erythemal reaction induced by the irradiation and the attenuation of the skin hypersensitivity test reactions; this was true for both delayed and immediate skin reactions in the case of UVB and for immediate skin reactions in the case of PUVA. The mechanism behind the attenuating effect of UV radiation on skin hypersensitivity reactions remains unknown, but it probably does not result from a stabilization of the mast cell membrane, as histamine and compound 48/80 induced reactions were suppressed to a similar extent.