The present study examined the antioxidant activity of conjugated octadecatrienoic fatty acid (9 cis, 11 trans, 13 trans-18:3), α-eleostearic acid, of karela seed (Momordica charantia), fed to rats for 4 wk. The growth pattern of rats and the effect on plasma cholesterol and high density lipoprotein (HDL) cholesterol and peroxidation of plasma lipid, lipoprotein, eryhrocyte membrane, and liver lipid were measured. Rats were raised on diets containing sunflower oil mixed with three different levels of conjugated trienoic fatty acid (9c, 11t, 13t-18:3) 0.5, 2, and 10% by weight; the control group was raised with sunflower oil as dietary oil as the source of linoleic acid (9c, 12c-18:2). The growth pattern of the three experimental groups of rats showed no significant difference compared to the control group of rats, but the group with 10% 9c, 11t, 13t-18:3 had slightly higher body weight than the control group of rats. Concentrations of total cholesterol, HDL-cholesterol, and non-HDL-cholesterol in plasma were similar in all four groups. Plasma lipid peroxidation was significantly lower in the case of 0.5% 9c, 11t, 13t-18:3 group than the control group and the 2 and 10% 9c, 11t, 13t-18:3 dietary groups as well. Lipoprotein oxidation susceptibility test with 0.5, 2, and 10% 9c, 11t, 13t-18:3 dietary groups was significantly less susceptible to lipoprotein peroxidation when compared with sunflower oil dietary group, and the dietary group with 0.5% 9c, 11t, 13t-18:3 showed least susceptibility. There was significant lowering in erythrocyte ghost membrane lipid peroxidation in the 0.5, 2, and 10% 9c, 11t, 13t-18:3 dietary groups compared to the sunflower oil groups. Nonenzymatic liver tissue lipid peroxidation was significantly lower in the group of rats raised on 0.5% 9c, 11t, 13t-18:3, but the groups on 2 and 10% 9c, 11t, 13t-18:3 acid did not show any significant difference compared with the control group of rats.