Exposure to combustion engine exhaust and its effect on crews of roll-on roll-off ships and car ferries and on bus garage staff were studied. The peak concentrations recorded for some of the substances studied were as follows: total particulates (diesel only) 1.0 mg/m3, benzene (diesel) 0.3 mg/m3, formaldehyde (gasoline and diesel) 0.8 mg/m3, and nitrogen dioxide (diesel) 1.2 mg/m3. The highest observed concentration of benzo(a)pyrene was 30 ng/m3 from gasoline and diesel exhaust. In an experimental study volunteers were exposed to diesel exhaust diluted with air to achieve a nitrogen dioxide concentration of 3.8 mg/m3. Pulmonary function was affected during a workday of occupational exposure to engine emissions, but it normalized after a few days with no exposure. The impairment of pulmonary function was judged to have no appreciable, adverse, short-term impact on individual work capacity. In the experimental exposure study, no effect on pulmonary function was observed. Analyses of urinary mutagenicity and thioether excretion showed no sign of exposure to genotoxic compounds among the occupationally exposed workers or among the subjects in the experimental study.