Dark mutants of the luminous bacteria Photobacterium fischeri reverted to hereditary stable luminescent forms when incubated with human polymorphonuclear neutrophils (PMN). The maximal mutagenic effect occurred during the first 15 min of phagocytosis, and was dependent on the phagocyte:bacterium ratio as well as on the integrity of the PMN cells. Heat-killed phagocytes or disintegrated phagocytes did not show any mutagenic activity, whereas the supernatant of the phagocytosis reaction exerted mutagenic activity. Scavengers of hydroxyl radical such as mannitol or benzoate and scavengers of singlet oxygen such as beta-carotene, as well as the presence of superoxide dismutase, prevented the mutations. The role of reactive oxygen metabolites in the phagocyte-mediated mutagenic process is discussed.