Because the cholesterol-rich very low density (VLD) lipoproteins of subjects with type III hyperlipoproteinemia are distinctively enriched in apolipoprotein E, a radial immunodiffusion assay for apolipoprotein E in whole plasma was developed. Its diagnostic usefulness was tested in randomly selected (n = 174) and hyperlipidemic (n = 61) subsets of an adult employee population and a hyperlipidemia clinic referral group (n = 63), which included 18 patients with well-documented type III hyperlipoproteinemia. Apolipoprotein-E levels were normally distributed among the random population subset, were equal between the two sexes, and increased little with age. The mean and 99th percentile values were 24.6 and 40.1 mg/dl, respectively. All subjects with type III patterns as assigned by standard criteria from both population (n = 4) and referral sources exceeded this 99th percentile (chi +/- SD = 54.7 +/- 9.7 mg/dl). Hence a plasma apolipoprotein-E concentration exceeding 40 mg/dl appears diagnostic of type III hyperlipoproteinemia, representing the first application of an apolipoprotein immunoassay to improved diagnosis of the hyperlipoproteinemias.