Studies were performed to investigate the effects of chronic, low level pre- and post-natal lead exposure on cell-mediated immune function in rats. Weanling female rats were exposed to lead (as lead acetate) in their drinking water at 0, 25, and 50 ppm for 7 weeks. At the end of 7 weeks they were mated with untreated males and continued on the same dosage throughout gestation and lactation. The offspring of these females were weaned at 21 days of age and continued on the same lead exposure regimen as their mothers. These offspring were used in immune surveillance procedures between 35 and 45 days of age. Lead exposure at the levels employed had no statistically significant effect on growth and did not result in overt signs of toxicity. Thymic weights were significantly decreased in both males and females of the two lead dosage groups. Furthermore, lead exposure resulted in suppression of responsiveness of lymphocytes to mitogen stimulation and in reduced delayed hypersensitivity responsiveness. Results indicate that chronic low-level lead exposure causes suppression of cell-mediated immune function.