The purpose of this study was to determine the concentrations and composition of apoB-containing lipoprotein families in whole plasma and major lipoprotein density classes of a selected group of Native Americans from northeastern Oklahoma with non-insulin-dependent diabetes mellitus. The measurement of lipoprotein density classes showed that the total lipoprotein mass of very-low-density lipoproteins was significantly higher and that of high-density lipoproteins significantly lower in diabetic patients than nondiabetic control subjects regardless of their plasma triglyceride levels. The VLDLs were enriched with TG, free cholesterol, and apolipoproteins C-III and E. HDLs were enriched with TG but depleted of apoC-III and apoE. There was no change in the levels of TG-enriched low-density lipoproteins. Fractionation of VLDL by sequential immunoprecipitation with antisera to apoE and apoC-III established that increased concentrations of this density class in diabetic patients are due to elevated levels of TG-rich lipoprotein LP-B:C and lipoprotein LP-B:C:E. The levels of LP-B:C particles were increased more than the levels of LP-B:C:E particles. The LDLs were characterized by a slight increase in TG-enriched lipoprotein B and no change in the levels of LP-B:C and LP-B:C:E. There was no difference between diabetic patients with or without vascular disease in the levels of LP-B and LP-B:C:E. However, patients with vascular disease had higher concentrations of LP-B:C particles in VLDL and whole plasma than patients without vascular disease. In conclusion, the most characteristic abnormalities of lipid transport in people with NIDDM are increased levels of TG-rich LP-B:C and LP-B:C:E particles of modified chemical composition and decreased concentrations of apoA-l-containing lipoproteins of modified chemical composition. These abnormalities seem to be present in most diabetic patients regardless of their plasma TG levels. It appears that diabetic patients with vascular disease have higher levels of LP-B:C particles and higher LP-B:C-LP-B:C:E ratios than diabetic patients without vascular disease. The metabolic behavior and atherogenic potential of LP-B:C and other TG-rich apoB-containing lipoproteins remain to be ascertained in future studies.