1. Large variations have been observed in the cholesteryl ester transfer activity of lipoprotein-free plasma isolated from 16 vertebrate species. 2. The different species have been arbitrarily divided into low, intermediate and high transfer activity groups, with man in the intermediate group. 3. Cholesteryl ester transfer activity did not correlate with the molar rate of cholesterol esterification in the plasma, nor with the concentration of cholesteryl ester in low density lipoproteins or high density lipoproteins. 4. It correlated positively and significantly with the concentration in very low density lipoproteins (r = 0.34, P = 0.014).