Reports of rare cases of suspected transmission of the human immunodeficiency virus (HIV) from mother to children by breast milk have been recently published. To study the factors that possibly limit HIV transmission through breast-feeding, milk samples obtained from 15 healthy, seropositive mothers and 4 seronegative control subjects were studied for the presence of anti-HIV antibodies. All samples from seropositive women contained IgG antibody against envelope glycoproteins gp160 and/or gp120, and 11 of 15 samples contained IgA antibodies against gp160. IgA antibodies against other viral antigens were more rarely recovered, except against the internal proteins of the virus, p18 and p25. The finding of IgA antibodies to HIV-1 in breast milk establishes that the virus elicits a local immune response in heterosexual, seropositive women. The role of local antibodies in the postnatal transmission of HIV remains to be determined.