Fatty acid composition of serum cholesteryl esters (CEs) in Chinese and 9 species of laboratory animals was analysed. Polyunsaturated fatty acids (PUFAs) were the predominant fatty acids in CEs. A close correlation between PUFAs to saturated fatty acids ratio (P/S) in CEs and susceptibility to atherosclerosis (AS) was observed in different species. Animals with high P/S such as mice (13.0), tree shrews (9.0) and rats (9.0) were AS-resistant, and those with low P/S such as rabbits (2.9) were susceptible to AS. The P/S value in men (4.9) was between those of the above two kinds of animals, and similar to those of pigs (4.2) and monkeys (3.7). Marked physiological significance of the rich PUFAs in CEs may be expected. As the main constituent of low density lipoprotein (LDL), CEs may provide PUFAs for cells efficiently via LDL receptor. It is assumed that PUFAs in CEs may play an important role in the metabolism of cholesterol and the development of AS by regulating LDL receptor activity, platelet function and prostaglandin metabolism.