The serum low density lipoprotein (LDL; p 1.019-1.050 g/ml) of the normal Macaca mulatta monkey (rhesus), kept on a low-fat Purina primate chow diet, was isolated by ultracentrifugal flotation, and its physicochemical properties were compared with those previously reported for human LDL. Rhesus LDL was found to be chemically similar to human LDL. The values for the sedimentation (S25, w-O) and diffusion (D25,w-O) coefficients were 7.09 S and 2.50 times 10- minus-7 cm-2 sec- minus-1, respectively. The intrinsic viscosity was 3.40 ml g- minus-1. The partial specific volume of rhesus LDL, determined in an Anton Paar precision density meter, was 0.960 ml g- minus-1. Molecular weights, calculated from a combination of S-O and D-O and of S-O and [n], were in agreement with the weight-average molecular weight, Mw, of 2.29 times 10-6 obtained by high-speed sedimentation equilibrium. In addition, a Z-average molecular weight, Mz, of 2.73 times 10-6 was calculated because curvature in the graphs of log c vs. r-2 indicated that rhesus LDL was heterogeneous. From the frictional ratio of 1.02, a maximum hydration of 0.1 g of H2O/g of lipoprotein was obtained. On electron micrographs, rhesus LDL appeared spherical with a mean diameter of 196 A, which was substantiated by hydrodynamic analysis.