Transplacental transfer of specific IgG antibodies was studied in 46 pairs of human immunodeficiency virus type 1 (HIV-1)-seropositive women and their neonates and in 53 pairs of healthy HIV-seronegative mothers and their newborns. Neonatal and maternal sera were assessed by nephelometry for total levels of serum IgG and by ELISA for IgG antibodies to herpes simplex virus (HSV), varicella-zoster virus (VZV), measles virus, tetanus toxoid, streptolysin O, and Streptococcus pneumoniae capsular antigens. Placental transfer of IgG antibodies to VZV, tetanus toxoid, measles, streptolysin O, and S. pneumoniae was decreased by maternal HIV infection. Maternal levels of total IgG had an independent effect on transfer of antibodies to HSV, VZV, measles, and S. pneumoniae. Neonatal antibody levels to tetanus toxoid, measles, and S. pneumoniae were significantly lower in the HIV group. Both maternal hypergammaglobulinemia and maternal HIV infection may contribute to these low antibody levels at birth and thus lead to early infection in this high-risk population.