In human immunodeficiency virus (HIV)-infected individuals the amount of antibodies formed after vaccination with T-cell-dependent recall antigens such as tetanus toxoid is proportional to the peripheral blood CD4⁺ T-lymphocyte counts. To investigate whether the immunoglobulin G (IgG) subclass distribution and avidity of the antibodies produced after vaccination are affected as well, we gave 13 HIV-infected adults with low CD4⁺ T-lymphocyte counts (<200 × 10⁶/liter; group I), 11 HIV-infected adults with intermediate CD4⁺ T-lymphocyte counts (≥200 × 10⁶/liter; group II), and 5 healthy controls booster immunizations with tetanus toxoid. The prevaccination antibody concentrations against tetanus toxoid were similar in the HIV-infected and healthy adults. After vaccination the total IgG and the IgG1 anti-tetanus toxoid antibody concentrations were significantly lower in group I than in group II and the controls. The avidity of the IgG1 anti-tetanus toxoid antibodies formed by HIV-infected adults was within the range for healthy controls, irrespective of their CD4⁺T-lymphocyte counts.