Many furocoumarins found in several species of plant are potent photosensitizing agents known to cause lethal and mutagenic effects in a wide range of organisms, from viruses to man. Their role in the aetiology of cancer is debatable, but work has focused on the PUVA (psoralen-UVA) treatment of psoriasis with 8-methoxypsoralen (8-MOP) and near UV radiation. Bergaptene (5-methoxypsoralen, 5-MOP) is a major constiutent of oil of bergamot, and might be expected to have qualitatively similar photosensitizing properties to 8-MOP. Although 5-MOP is widely used as a stimulus to melanin deposition in several suntan preparations surprisingly little is known about its basic photobiology. We report here that 5-MOP has the expected properties of other biologically active furocoumarins. These properties include lethal and mutagenic photosensitization of bacteria, 'dark' induced frameshift mutagenesis in bacteria, and lethal and clastogenic effects on mammalian cells in tissue culture.