In a repair shop for train engines a pilot study was conducted to investigate occupational exposure to diesel exhaust. 1-Nitropyrene was determined in stationary sampled total suspended particulate matter collected on 2 consecutive workdays. Air concentrations of particulate associated 1-nitropyrene varied from non-detectable to 5.6 ng/m3. In spot urine samples collected on Sunday, Monday and Tuesday urinary metabolites of polycyclic aromatic hydrocarbons and their nitro-substituted derivatives were determined using an immunoassay. In the urine samples of 3 diesel mechanics both cumulative and average excretion of urinary metabolites over 48 and 72 h were significantly enhanced (P < 0.05) as compared to the excreted levels in urine samples from 2 office clerks.