Differences among laboratory animal species in the pulmonary carcinogenicity of chronic inhalation exposure to diesel exhaust have raised several important interpretive issues. Under similar heavy exposure conditions, it is clear that diesel exhaust is a pulmonary carcinogen in rats, but not in Syrian hamsters. Previous reports give conflicting views of the response of mice, which is presently considered equivocal. This report describes carcinogenicity results from a bioassay of CD-1 mice conducted in parallel with a previously reported bioassay of F344 rats (Mauderly et al. (1987) Fundam. Appl. Toxicol. 9, 208-221). Exposure to whole diesel exhaust 7 hr/day, 5 days/week for 24 months at soot concentrations of 0.35, 3.5, or 7.1 mg/m3 caused accumulations of soot in mouse lungs similar to those in lungs of rats and, like the results from rats, did not significantly affect survival or body weight. In contrast to the dose-related neoplastic response of rats, however, the exposures of mice did not increase the incidence of lung neoplasms. This finding is consistent with other data showing that mice, as well as Syrian hamsters, differ from rats in their lung neoplastic and nonneoplastic responses to heavy, chronic inhalation exposure to diesel exhaust soot and several other particles. Although rodents serve as useful indicators of potential human carcinogenic hazards, it is not yet clear which, if any, rodent species have lung neoplastic responses that are useful for quantitative predictions of human lung cancer risk from chronic inhalation of poorly soluble, respirable particles.