The levels of aromatic DNA adducts were compared by the 32P-postlabeling assay between the lymphocytes isolated from bus maintenance and truck terminal workers, using hospital mechanics as a control group. The adduct levels were elevated in all the bus and truck terminal workers. Within the bus maintenance personnel, garage workers had the highest levels of adducts. Within the terminal workers those driving diesel forklifts had the highest adduct levels. The results suggest that diesel exhaust contributes to the level of adducts.