The effects of low level, chronic polychlorinated biphenyl--Aroclor 1254--(PCB) exposure were investigated on non-specific immune parameters in female rhesus (Macaca mulatta) monkeys. Five groups of monkeys were orally administered with PCB at concentrations of 0, 5, 20, 40 or 80 micrograms/kg bw/day. Immunotoxicity testing was initiated after 55 months of exposure. The serum hemolytic complement activity in all PCB treated groups was significantly higher (P less than 0.05) than that in the control group. A statistically significant dose-related increase in natural killer cell activity was evident at the 75:1 effector to target cell ratio. Similarly, a statistically significant dose-related increase was noted for thymosin alpha-1 levels but not for thymosin beta-4 levels. Statistically significant increased interferon levels were noted in the 20 and 80 micrograms/kg groups compared with the control group while the levels in the 40 micrograms/kg group were decreased significantly compared with the control group. The production of tumor necrosis factor by monocytes in the PCB treated groups was not different to that in the control group. The results indicated that long term exposure to PCB modulate several non-specific immune parameters.