The effects of PCB (Aroclor 1254) on the immune system of adult female rhesus monkeys were investigated in a chronic study wherein five groups of monkeys (16/group) were administered (orally) PCB at levels of 0.0, 5.0, 20.0, 40.0 or 80.0 micrograms/kg body wt daily. Tests for immunotoxicity were initiated at 23 months of exposure to PCB, at which time the monkeys had achieved an apparent pharmacokinetic steady state based on the PCB concentration in fat and/or blood. A statistically significant (P less than 0.05) dose response reduction in antibody levels (IgG and IgM) to sheep red blood cells (SRBC) was observed following i.v. administration of three immunizing doses of SRBC at weekly intervals. A statistically significant decrease in the percent TH and an increase in the percent and absolute TS lymphocyte levels was found in the 80 micrograms/kg body wt group compared to the control. The TH/TS ratio was also significantly lower in the 80 micrograms/kg body wt group compared to the control. Other parameters investigated including percent of B-lymphocytes and total T-lymphocytes, total serum immunoglobulin levels (IgG, IgM and IgA), other serum proteins, glucocorticosteroid levels and lymphocyte transformation results following stimulation with the mitogens PHA-P and Con A were not affected significantly by PCB treatment. Additional immunologic parameters are currently being investigated to further elucidate the mechanism by which PCB induces immunotoxicity.