Erythrosine increased the yield of sporulation minus mutants of Bacillus subtilis excision repair-proficient strain 168 by approximately equal to 400% at a concentration of 1 mg/ml under ambient light conditions. This mutagenic response was dose-dependent (0-1 mg/ml). Significantly, the food dye did not mutate the corresponding repair-deficient B. subtilis, hcr-9 (exc-). A decrease in the mutagenicity of erythrosine to B. subtilis strain 168 was apparent on metabolism by rat liver S9 mixture or by rat caecal cell-free extracts. Erythrosine did not exhibit differential toxicity to B. subtilis excision repair-proficient (168) and -deficient (hcr-9) strains, although it was highly toxic to both strains. This indicated non-involvement of excision repair in the dye-mediated toxic reactions.