Photolysis of arylazides produces short-lived reactive species, very likely arylnitrenium ions which bind to nucleotides and DNA and produce mutations in Salmonella. The present report shows that arylazides can be photo-activated in mammalian (V79 Chinese hamster) cells and that sister chromatid exchange can thus be induced. Arylazides studied are (in order of decreasing SCE-inducing potency) azido-isoIQ, azido-MeIQ, azido-IQ, azido-MeIQx, azido-PhIP, 6-azido-chrysene, 2-azidofluorene, 4-azidofluorene, 2-azido-naphthalene, 4-azidobi-phenyl, 2-azidobiphenyl, 2,4,6-trimethylphenylazide, phenylazide (inactive). The structure-activity relationships emerging from the data are the same as those found previously in Salmonella. In line with this, a clearcut positive linear correlation was seen between the logarithm of the SCE-inducing potency in V79 cells and the logarithm of the mutagenic potency in Salmonella (r = 0.955). Therefore, the ultimate reactive species derived from IQ and related heterocyclic aromatic amines are extremely potent genotoxins, not only in a bacterial but also in a mammalian cell. Previous findings of only weak genotoxic activity of IQ and related food mutagens in certain cultured mammalian cells must therefore be reinterpreted as the result of an insufficient activation of these amines in the cells used, possibly because of insufficient acetylation competence.