Abstract High density lipoproteins (HDL) were isolated by centrifugation from the serum of salmon (Oncorhynchus gorbuscha) captured just before spawning. There were no detectable low density lipoproteins in their serum. Analytical ultracentrifugation of the lipoprotein fraction yielded only one band with a hydrated density of 1.103 g per ml and a peak F1.20 rate of 3.21 S. If the molecule is spherical, it would have minimum molecular weight of 180,000 and an average diameter of 80 A. The agarose gel electrophoresis pattern had one major and one minor band with the approximate mobility of human α-lipoprotein. The lipid class composition was very similar to that reported for human HDL. The phospholipids were mainly phosphatidylcholine (80%), its lysoderivative, and sphingomyelin. The major fatty acids were 16:0, 18:0, 18:1 (n-9), 20:5 (n-3), and 22:6 (n-3). Lipids accounted for 60% of the mass of the molecule. Disc electrophoresis in polyacrylamide gels (in 8 m urea at pH 8.6) produced a pattern with two major and several minor bands for both the intact lipoproteins and the lipid-free protein moiety. The over-all pattern was very similar to that of human HDL. The apoproteins from the salmon HDL were eluted from diethylaminoethylcellulose columns under conditions similar to those used for elution of apoproteins from human HDL; they also gave similar elution patterns. Their circular dichroism spectra resembled those of the corresponding human apolipoproteins. Despite all of these physical similarities, there were distinct and major differences between the amino acid compositions of salmon apoproteins and their human counterparts. These data suggest that general physical properties of the HDL are not specifically dependent on the total amino acid composition of the molecule but rather on its general secondary or tertiary structure and these same features must govern the binding of lipids to proteins in soluble lipoproteins.