To investigate the influence of 8-MOP and UVA on the induction of cell-mediated immunity, guinea pigs, sensitized with a single injection of dinitrofluorobenzene (DNFB) in Freund's complete adjuvant (FCA) in all footpads and the nuchal skin, were treated with 8-MOP, UVA, or 8-MOP + UVA on days 0, 2, and 5 and tested with dinitrochlorobenzene (DNCB) on day 14 after sensitization. Two control groups, exposed in a covered condition to the PUVA 4000 lamp, to observe the heat effect, showed a slightly enhanced contact sensitivity in comparison to the only sensitized control group. No altered reactivity was observed after irradiation with longwave UV light alone, whereas a statistically significant enhanced resp. reduced contact sensitivity was obtained after the treatment with 8-MOP alone and the combined action of 8-MOP + UVA, respectively.