Numerous physiological effects are attributed to conjugated linoleic acid(CLA). The purpose of this study is to consider these effects with respect to the cis-9, trans-11 and trans-10, cis-12 CLA isomer. Both isomers are natural products. The c9, t11-CLA isomeris the principal dietary form of CLA, but the concentrations of this isomer and the t10, c12-CLA isomer in diary product or beef vary depending on the diet fed to cows or steers, respectively. The influence of dietary CLA isomers on the immune response was examined, body weight and weight ratio of organ to body of Balb/c mice. Mice were divided into four groups of 8 mice. Balb/c mice were fed the experimental diets supplemented with 1% CLA(c9, t11-CLA isomer : t10, c12-CLA isomer = 2:3) (Group 1), 1% CLA(c9, t11-CLA isomer : t10, c12-CLA isomer = 1: ) (Group 2), 1% safflower oil (Group 3) or nothing (Control) for 3 weeks. After 3 weeks, serum, gut lumen lavage, fat, liver, spleen and thymus were taken. Measurement of total immunoglobulin were executed using sandwich ELISA. Serum levels of IgA and IgM showed that group fed with t10, c12-CLA isomer significantly were higher than group fed with c9, t11-CLA isomer. In addition serum level of IgF showed that group fed with t10, c9-CLA isomer significantly were lower than group fed with c9, t11-CLA isomer. However, no significantly differences were observed in the serum IgE and secretory IgA. Weight ratio of spleen to body showed no significant differences. In weight ratio of liver and thymus to body, t10,c9-CLA isomer significantly were respectively higher than group fed with c9, t11-CLA isomer. In conclusion, t10, c12-CLA isomer produced a situation favorable for immunopotentiative effect and body composition. But it should be protected against hepatomegaly induce lipic accumulation in liver.