Human IgG subclasses differ in their biologic functions and are restricted as specific antibodies to certain Ag. The basis for this restriction is unknown but in order to characterize it further, we obtained serum preparations containing one single subclass by using subclass-specific mAb in affinity chromatography. Subsequently we determined the affinity of antibodies in the four IgG subclasses for gp30/p25, a hepatitis B surface Ag (HBsAg) complex, for a nine-amino acid cyclical peptide representing residues 139 to 147 of HBsAg, and for a peptide representing residues 126 to 140 of the hepatitis B virus pre-S2 region and the relative avidity of IgG1 and IgG2 antibodies for purified pneumococcal polysaccharide type 3. Affinities to the HBsAg showed a clear pattern of decreasing affinity in the order IgG1 greater than IgG2 greater than IgG3 greater than IgG4 both in sera from vaccinated and from naturally infected individuals. The relative avidities of antibodies to the polysaccharide Ag had a reverse pattern, IgG2 greater than IgG1. In individuals with or without Ig H chain gene deletions where the anti-HBsAg response was restricted to one subclass, the affinity was similar to that observed for the same subclass in sera from individuals who in addition possessed high or low affinity antibodies of other subclasses.