Patas monkey lipoproteins were fractionated into four distinct classes by a combination of ultracentrifugation and Geon-Pevikon block electrophoresis and characterized with respect to their chemical and physical properties. Very low density lipoproteins (VLDL) were isolated at d is less than 1.006, were triglyceride rich, and were in the size range 300-850 A. They were similar in apoprotein content to the VLDL of man, dog, and swine. The Patas monkey low density lipoprotein referred to as LDL-I had beta mobility and a size which ranged from 190 to 240 A in diameter. Their chemical composition and apoprotein content were similar to those of human LDL. A second low density lipoprotein referred to as LDL-II occurred at a density of 1.05-1.085, ranged in size from 190 to 300 A, and contained the B, arginine-rich, and A-I apoproteins. Differences between LDL-I and LDL-II included a higher sialic acid content for LDL-II and lipid to protein ratios of 3.7 and 3.0 for LDL-I and LDL-II, respectively. In addition, the LDL-II, but not LDL-I, reacted immunochemically with antisera prepared to human Lp(a). The physical, chemical, and immunochemical properties indicated that monkey LDL-II were equivalent to the human Lp(a). Patas monkey HDL, equivalent to human HDL, were protein and phospholipid rich and ranged in size from 70 to 100 A in diameter. The two major HDL apoproteins, A-I and A-II, were isolated from apo-HDL by column chromatography. The amino-terminal sequence of Patas A-I showed striking homology to that reported for human, dog, and swing A-I. The amino acid composition of monkey A-II was very similar to that of human A-II; however, unlike human A-II, the monkey apoprotein was shown to exist as a monomer similar to that reported for Rhesus monkey A-II. The similarities between the plasma lipoproteins of the monkey and of man suggest that the Patas monkey would serve as a suitable model for metabolic studies.