Two current definitions of type III hyperlipoproteinemia, "floating beta" lipoproteins and an estimate of the relative content of cholesterol and triglyceride in lipoproteins of density less than 1,006 (very low-density [VLD] lipoproteins), have been compared. Over 3100 complete lipoprotein analyses of 182 adults with primary familial hyperglyceridemia, covering a wide range of plasma lipid concentrations, formed the data base for this retrospective analysis: The ratio of VLD lipoprotein cholesterol to the plasma triglyceride concentration (VLDL/TG=r) proved capable of segregating an apparently unique subpopulation with persistently abnormal very low-density lipoprotein composition. Although floating beta lipoproteins were present in nearly all patients with a high r value, they also appeared inconsistently in many other patients. It is concluded that the chemical index to VLD lipoprotein composition is the better, albeit, temporary definition for this disorder. When the plasma triglyceride concentration is between 150 and 1000 mg/100 ml, an r not less than 0.25 should be considered as suggestive and a value not less than 0.30 as diagnosttc of type III hyperlipoproteinemia.