Benzo[a]pyrene (BaP) is a useful indicator of exposure to polycyclic aromatic hydrocarbons (PAHs), airborne carcinogenic compounds. Radioimmunoassay was used to test for plasma BaP differences in 61 subjects divided into three groups based on geographic-demographic locale: urban-industrial, urban-residential, and outer suburban. The results showed that the urban-industrial area participants had a significantly higher mean plasma BaP level than did the outer suburban subjects. The urban-residential subjects did not have a significantly different mean plasma benzo[a]pyrene level from either of the other two groups. Obesity, as measured by Quetelet's index, was found to have a significant correlation with BaP levels. These results indicate that radioimmunoassay of plasma for BaP may be used successfully to judge environmental exposure to PAHs, provided physiological considerations such as obesity are taken into account.