Well-known skin photosensitizers furocoumarins--psoralen, angelio in and 8-methoxypsoralen were shown to photosensitize oxidation of unsaturated lipids and dihydroxyphenylalanine via a two-step mechanism. At the first step (UV-irradiation at 366 nm) photooxidation of furocoumarins occurred, at the second step--the photooxidized furocoumarins oxidized unsaturated lipids or dihydroxyphenylalanine in darkness. The photochemical activity of furocoumarins in the two-step reaction decreased as follows: psoralen greater than angelicin greater than 8-methoxypsoralen. Similar photoreactions appear to participate in induction of phototoxic effects of furocoumarins.