To test whether beta-migrating very low-density lipoproteins (VLDL, d less than 1.006) might not be unique to broad-beta disease (with a Type III lipoprotein pattern) but rather a caricature of an intermediate species in the catabolism of triglyceride-rich lipoproteins of normal composition and electrophoretic mobility in nonretarding media, VLDL from subjects with endogenous hypertriglyceridemia (with a Type IV pattern) or broad-beta disease were analyzed under varying dietary and pharmacologic conditions following starch-block electrophoresis. These studies revealed a spectrum in electrophoretic mobility and lipid composition throughout the Sf20 to 400 range: the more buoyant, triglyceride-rich VLDL migrated faster and the denser, triglyceride-poor VLDL more slowly, but the VLDL were broadly and continuously distributed throughout the entire beta to alpha2 regions in both disorders. However, in each subfraction of VLDL (Sf100 to 400, 60 to 100 and 20 to 60) as well as in the whole Sf20 to 400 class, the relative proportion of slower species was greater in the subjects with broad-beta disease than in those with endogenous hypertriglyceridemia. Under conditions of acutely stimulated VLDL production (following an oral fat load), a late increase in the slower species was observed as alimentary lipemia resolved. During chronic VLDL hypersecretion (with high carbohydrate feeding) both faster and slower species increased in a subject with broad-beta disease. In the same subject during clofibrate therapy, the faster species were decreased more than the slower on both normal and high carbohydrate diets. Acute acceleration of VLDL catabolism by heparin administration increased the slower VLDL at the expense of the faster, both in this subject and in a counterpart with endogenous hypertriglyceridemia. These studies are consistent with the hypothesis that slower migrating, triglyceride-poor VLDL are normal intermediate (or remnant) forms in a continuous catabolic process. The concentration of these remnants is dwarfed by that of the faster species in subjects with endogenous hypertriglyceridemia. However, in subjects with broad-beta disease they accumulate as the beta-VLDL characteristic of this disorder, most likely as a result of a relative blockade in their further catabolism.