Furocoumarins are phototoxic and photomutagenic natural plant constituents found in many medicinal plants and food items. Because plants contain mixtures of several furocoumarins, there is a need for a comparative risk assessment of a large number of furocoumarins. Little is known about the photomutagenicity of the structurally related family of coumarins, which are also abundant in many plant species. In this study, we analyzed the photomutagenic potency of the linear furocoumarins 5-methoxypsoralen (5-MOP) and 8-methoxypsoralen (8-MOP), the angular furocoumarin angelicin, and the coumarin limettin. Above certain concentrations, all test compounds were more or less phototoxic in the presence of UVA doses between 50 and 200 mJ/cm(2), 5-MOP being the most phototoxic compound. At nonphototoxic concentrations, linear correlations were found between concentration and mutagenicity at a UVA dose of 125 mJ/cm(2) for all test compounds including limettin. For 5-MOP, strictly linear correlations were also found for the relationships of mutagenicity vs concentration at various UVA doses or vs UVA dose at given concentrations, respectively. These data indicate that the photomutagenicity of 5-MOP is proportional to the UVA dose x concentration product for noncytotoxic combinations of both factors. They also suggest that the slope of the concentration-photomutagenicity correlation at a given UVA dose may provide a basis for comparison between individual compounds. Applying this concept, in vitro photomutagenicity equivalency factors at 125 mJ/cm(2) were as follows: 1.0 (5-MOP, reference compound), 0.25 (8-MOP), and 0.02 (angelicin and limettin, respectively). These findings provide a new concept for the description of the relative photomutagenic potency of coumarins and furocoumarins and indicate that, in V79 cells, 8-MOP is less photomutagenic and limettin and angelicin are much less photomutagenic than 5-MOP.