The aim of this study was to determine the influence of dietary supplementation of conjugated linoleic acid (CLA) on some immune system parameters in piglets. Forty-two weaned piglets, 12.44 +/- 1.96 kg liveweight, were fed diets containing 1% sunflower oil (control, C), 0.5% CLA plus 0.5% sunflower oil (T1), and 1% CLA (T2). After 28, d serum lysozyme (1.39, 1.77 and 1.86 mug mL(-1), respectively, for C, T1 and T2 groups; P< 0.01) and total serum immunoglobulin G (IgG) (1548, 1930, 2075 mg dL(-1), respectively, for C, T1 and T2 groups; P< 0.05) were higher in piglets fed dietary CLA. No significant effect on serum alpha-1-acyl glycoprotein (AGP) was observed. These results support the view that CLA favorably influences immune response.