Air pollutant dusts as well as chemically defined particles were examined for their activating effect on oxidant production (O2- and H2O2) in guinea pig alveolar macrophages (AM). Oxidant production was measured as chemiluminescence of albumin-bound luminol. All particles examined stimulated the AM in a dose-dependent manner to different maximal levels of oxidant production. Amphibole asbestos samples were the most active of all agents studied. Various immune reactants as well as silica, metal-oxide-coated fly ash, polymethyl methacrylate beads, and chrysotile asbestos had intermediate activity, while fugitive dusts, polybead carboxylate microspheres, glass and latex beads, uncoated fly ash, and fiberglass had the lowest activity. In addition to direct stimulatory action on AM, particles also lowered the subsequent responsiveness of the cells to the bacterial peptide stimulant, N-formylmethionyl phenylalanine. This effect was only partially due to the cytotoxicity of the particles. While some relationship appeared to exist between stimulatory activity and cytotoxicity of the particles, the exact role of O2- in mediating cytotoxic effects is still open to question. Oxidant production in AM is a parameter which may be important in determining the pathological effects of particles as well as of materials absorbed to their surfaces.