The ability of particulate air pollutants (and possible constituents) to alter pulmonary host defenses was examined using an in vitro alveolar macrophage cytotoxicity assay and an in vivo bacterial infectivity screening test which employed intratracheal injection of the particles. A wide range of response between particles was seen at the 1.0 mg/ml level in vitro and the 0.1 mg/mouse level in vivo. A sample of fluidized-bed coal fly ash, bentonite, asbestos, some ambient air particles and heavy metal oxides greatly increased susceptibility to pulmonary bacterial infection. Most coal fly ash samples and some air particles caused moderate increases in infectivity, while diesel particulates, volcanic ash, and crystalline silica caused only small increases. Cytotoxic effects on macrophages in vitro were observed with most of the particles. The in vivo and in vitro assays produced a similar ranking of toxicity, however, not all particles that were highly cytotoxic were potent in increasing bacterial infectivity. Increased toxicity measurable by either assay often appeared to be associated with small size or with the presence of metal in the particles. (Copyright (c) 1985 by Academic Press, Inc.)