IgG subclasses in anti-HBs antibodies in sera from 9 vaccinated (H-B-Vax, Merck, Sharp & Dohme) individuals and from 18 individuals with naturally acquired anti-HBs were characterized in enzyme-linked immunosorbent assay with monoclonal subclass-specific antibodies. Also eight hyperimmune immunoglobulin preparations from five producers were investigated. In vaccinated persons, the activity was predominantly present in IgG1 and IgG4 subclass. Various combinations of subclasses in anti-HBs antibodies were found in persons after recovery from hepatitis B and in immunoglobulin preparations. The serum concentration of subclasses does not reflect the subclass profiles of the anti-HBs antibody.