Heterogeneities in the density of hepatitis C virus RNA-carrying material (HCV-RNA-CM) found in human sera (1.03-1.20 g/cm3) are attributed to the binding of low-density lipoproteins and/or of IgG. In some sera HCV-RNA-CM seems to be nearly totally bound to beta-lipoproteins and cannot be precipitated by anti-IgG (gamma); in others more than 95% of HCV-RNA-CM is bound to IgG and cannot be precipitated by anti-beta-lipoprotein. Furthermore, there are sera from which HCV-RNA-CM can be completely be precipitated by either anti-beta-lipoprotein or anti-IgG (gamma), pointing to a binding of the two serum proteins to the same HCV-RNA-CM. There are other sera from which HCV-RNA-CM can be partially precipitated by the one or the other antiserum, leaving behind fractions, which are bound to beta-lipoprotein or to IgG. HCV-RNA-CM cannot be precipitated from some sera either by anti-beta-lipoprotein or by anti-IgG (gamma).