A new in vitro test for phototoxicity has been developed. Nine substances (eight photosensitizers and one non-photosensitizer) were screened for their ability to activate human complement in the presence of UV light, and all photosensitizers were found to be active. Complement activation was quantified by means of ELISA. In all cases activation was mediated by alternative, but not by classical, pathways. The results of a cell culture test performed with 5-methoxypsoralen suggest the existence of two different mechanisms of phototoxicity. This indicates that the battery of in vitro assays for phototoxicity prediction must contain at least two different tests.