Polycyclic aromatic hydrocarbons (PAHs) are environmental carcinogens exhibiting potent immunosuppressive properties. In order to determine PAH-induced immunotoxicity in humans, we investigated possible immunomodulating effects on T-lymphocyte proliferative responses and natural killer (NK) cell activities, at two different exposure levels, in asphalt and coke oven workers. We evaluated the efficiency of urinary 1-hy droxypyrene as a measure of exposure to PAHs. We found a statistically significant inhibition in T-lymphocyte proliferative responses of asphalt and coke oven workers compared to the controls. On the other hand, interestingly, we found significantly higher NK cell activities at three effector:target (E:T) ratios in the asphalt group compared to coke oven and control groups. We conclude that PAHs may cause suppression of T-lymphocyte proliferation at both exposure levels and augment NK cell activity only at low levels of exposure. Our results are in line with others reported in the literature indicating that chronic exposure to PAHs at different levels may alter some immune responses in different ways.