There is an increased risk of developing neoplastic disease at sites of chronic inflammation. We have used a model of rat pleural cavity inflammation induced with carrageenan to obtain inflammatory cells comprising predominantly phagocytes (macrophages, monocytes and neutrophils) to examine their ability to activate chemical carcinogens. Treatment of these cells with phorbol-12-myristate-13-acetate (PMA, 15 mM) to stimulate respiratory burst, resulted in a rapid release of reactive oxygen species. Incubation of the cooked food promutagen 2-amino-3,8-dimethylimadazo-[4,5-f]quinoxaline (MeIQx) with PMA (or phorbol dibutyrate or opsonised zymosan or silica) stimulated pleural cavity phagocytes, generated highly electrophilic products which were mutagenic in an Ames Salmonella mutagenicity assay. whereas resting cells (no PMA) had negligible activity. Mutagenic activation of MeIQx by PMA stimulated cells could be reduced by inhibitors of active oxygen such as mannitol, benzoate, dimethyl sulphoxide, superoxide dismutase and catalase and by inhibition of myeloperoxidase activity. In contrast the very potent, broad spectrum cytochrome P450 inhibitor 8-methoxypsoralen had no effect, suggesting the reaction was not cytochrome P450 dependent. Activation of MeIQx by PMA stimulated cells could be abolished by the protein kinase C inhibitor staurosporin, confirming the importance of protein kinase C in the reaction. Furthermore, performing incubations in the presence of fluoride (10 mM) to directly stimulate adenylyl cyclase avoided the requirement for phorbol ester in the activation process. These data show that phagocytes stimulated to release active oxygen species can activate MeIQx to a mutagenic derivative. Mutagenic activation of MeIQx in this system was dependent upon the generation of active oxygen via signal transduction pathways involving protein kinase C and adenylyl cyclase. We suggest that the mechanism of MeIQx activation by phagocytes probably involves one electron oxidation mediated by active oxygen species.