An immunoassay was developed for determining the concentration of the protein moiety of the low-density lipoproteins of human plasma. The concentration of this protein in the plasma was variable; it was higher than normal on the average in patients with familial hyperbetalipoproteinemia (type II) and endogenous hyperlipemia (type IV) and lower than normal in patients with fat-induced (type I) and mixed (type V) hyperlipemia. Patients with endogenous hyperlipemia were separable by the immunoassay into those with normal and those with supernormal low-density lipoprotein protein concentration.