Conjugated dienoic derivatives of linoleic acid(CLA) are a mixture of positional and geometric isomers of linoleic acid(LA). We previously found that CLA changes the fatty acid profile in chicken eggs and serum by decreasing monounsturated fatty acids. Studies were conducted to explore the effects of CLA on fatty acid composition. Rabbits were fed a semisynthetic diet with or without CLA(0.5g CLA/rabbit/day) for 22 weeks. Compared to the control, rabbits fed CLA had significantly lower monounsaturated fatty acid levels(palmitoleic acid C16 : 1 by 50% and olelc acid C18 : 1, by 20%) in plasma lipids. We found similar differences in fatty acid composition in the liver and the aorta. The inhibitory effect of CLA on $Delta{9}$ desaturation was confirmed in a human hepatoma cell line, Hep G2. CLA significantly decreased $Delta{9}$ desaturation in 4-5 hours as shown by an increase in the ratio of Cl6 : 0 to C16 : 1. This is apparently due to a decrease in $Delta{9}$ desaturase(stearoyl-CoA desaturase, SCD) activity ; it was decreased more than 50%. These results, along with our previous findings, indicate that CLA is an inhibitor of $Delta{9}$ desaturase in the liver.