The purpose of the study was undertaken to determine the effect of PCBs and PCDFs on serum protein components of rats. Four groups of 10 rats each were fed the diet containing 0 (controls), 1 ppM, 10 ppM of PCDFs or 100 ppM of PCBs. The rats fed the diet containing both doses of PCDFs showed a significantly depression of growth rate. Slight depression of growth rate was recorded for the rats fed the diet containing 100 ppM of PCBs. Absolute and relative weights of thymus were significantly decreased in the rats fed the diet containing both doses of PCDFs, while PCBs produced minimal effect on thymus weight. Leucocyte counts did not differ between controls and PCB- or PCDFs. Alpha 1-Globulin was significantly decreased in the rats fed the diet containing PCBs or PCDFs. Beta-Globulin was significantly increased in all treated groups. Gamma-Globulin was increased in rats fed the diet containing 100 ppM of PCBs or 10 ppM of PCDFs. Albumin, alpha 2-globulin and albumin-globulin ratio did not differ between controls and treated groups. (CAJ)