The plasma of rhesus monkeys which were fed a normal chow diet was shown to contain several low density lipoproteins of which three major species, namely, LDL-I, LDL-II, and LDL-III were isolated by a combination of isopycnic and rate zonal density gradient ultracentrifugation. Of the 30 animals tested, 28 contained LDL-I and LDL-II, and 15 had, in addition to the first two components, LDL-III. Several exhibited additional minor LDL components. LDL-III with a mean buoyant density of 1.050 g/ml had an unexpectedly large molecular weight (3.47 x 10/sup 6/) when compared to LDL-I (3.32 x 10/sup 6/) and LDL-II (2.75 x 10/sup 6/) which had buoyant densities of 1.027 and 1.036 g/ml, respectively. The unusual physical properties of LDL-III were found to be due to a higher content and nature of its protein moiety, represented only by apo B according to electrophoretic criteria and radioimmunoassay. The protein of LDL-III (apo LDL-III) also differed from the other two in having a higher galactose and sialic acid content. Moreover, relative to LDL-I and LDL-II, LDL-III exhibited differences in behavior when using techniques of circular dichroism, radioimmunoassay, agarose- and sodium dodecyl sulfate-gel electrophoresis, which could be attributed to its protein moiety. In contrast,more » the molar lipid content of LDL-III was almost identical to that of LDL-I, and differential scanning calorimetry showed that the neutral lipids of the three LDLs undergo almost identicaltransitions below body temperature. When all of the data were analyzed according to the method of Shen et al., the major structural difference between LDL-III and the other two LDLs appeared to relate to the degree of occupancy and conformation of the apoprotein in the surface of the particles.« less