We report on the physicochemical properties of unusual lipoproteins isolated from both lymph and blood of ruminating cattle. The densities of most of these particles fall within the range between 1.006 and 1.020 g/ml, although densities of 0.97-0.99 g/ml are calculated from chemical composition, assuming a liquid core. The triglycerides of these particles have a high content of saturated fatty acids. The major apoprotein has a mobility on polyacrylamide-SDS gels consistent with a molecular weight of 40,000. The negatively-stained particles appear flattened and asymmetric in electron micrographs. The particles are very large, with molecular weights in the 20 to 250 million dalton range, and they scatter light strongly. The hydrodynamic frictional ratio is about 1.4, consistent with oblate ellipsoids with axial ratios of about 8 to 1. The flat appearance, asymmetric shape, and anomalous densities of the particles would be explained if these lipoproteins consisted of a core of crystallized triglycerides encapsulated within a phospholipid monolayer. Crystallization of the saturated triglycerides could occur during routine lipoprotein isolation, in which temperatures much lower than the melting points of their core lipids are employed. when protocols are done entirely at 37 degrees C, the unusual structures are not observed in the intermediate density class. Although the saturated fats in these bovine lipoproteins are derived from ruminal fermentation, we feel that any triglyceride-rich lipoprotein highly enriched in saturated fats will behave similarly if isolation temperatures are well below the melting points of the core lipids.