Qualitative, quantitative, and comparative aspects of the serum lipoprotein profile in the Common marmoset (Callithrix jacchus), a New World primate, are described. Density gradient ultracentrifugation was used to evaluate lipoprotein distribution and to establish criteria for isolation of discrete molecular fractions. The major lipoprotein classes banded isopycnically on the gradient with the following hydrated densities: VLDL, d less than 1.017 g/mL; LDL, d = 1.027--1.055 g/mL; HDL fraction I, d = 1.070--1.127 g/mL; and HDL fraction II, d = 1.127--1.156 g/mL. Electrophoretic, immunological, and electron microscopic analyses attested to the purity of these fractions: the characteristics of each were assessed by chemical analysis, electron microscopy, immunological techniques, and polyacrylamide gel electrophoresis of their protein moieties. Marmoset VLDL and LDL were closely akin to those of man in size and chemical composition, although the former were richer in triglyceride; electrophoretic and immunological data showed the major protein component of VLDL and LDL to be a counterpart to human apo-B. The two HDL subfractions, i.e., HDL-I and HDL-II, corresponded in size and chemical composition to human HDL2 and HDL3, respectively, although slight differences in neutral lipid content were detected. By immunological and electrophoretic criteria, the major apolipoprotein of marmoset HDL was analogous to human apo-AI. In contrast, marked dissimilarities were evident in the complements of low molecular weight, tetramethylurea-soluble polypeptides of marmoset and human lipoproteins. Quantitatively, the human and marmoset lipoprotein profiles were not dissimilar, although HDL was the major class (approximately 50%); in fasting animals, serum concentrations of VLDL, LDL, and HDL were 50--90, 170--280, and 338--408 mg/dL, respectively. C. jacchus was distinct from man in displaying a greater proportion of its total HDL in the less dense (HDL-II) subfraction (marmoset HDL-I/HDL-II = approximately 4:1; human HDL2/HDL3 = approximately 1:3). These data indicate that, as an experimental animal for lipoprotein research, the Common marmoset combines the advantages of ready availability and maintenance with a serum lipoprotein profile which resembles, in many qualitative and quantitative aspects, that found in man.