Children born to HIV-1-positive mothers were prospectively tested for HIV-reactive IgG, IgM, and IgA by Western blot, in order to study the children's humoral immune response in the background of passively transferred maternal IgG. In infected infants, a response was first seen at 1–3 months forenv-reactive IgM and IgA, as well asgag-reactive IgM and IgG. This was followed by production of IgG toenv, IgA topoland togagp17 and p55 at 7–9 months, and IgG topolat 10–12 months. IgG Western blot positivity by all interpretation guidelines in all infected infants was found by 10–12 months. Subsequently, only IgG toenvand p24, and IgA toenvwere maintained in all, whereas IgG topoland p17 disappeared again in a significant fraction. A considerable proportion of uninfected infants also producedgag-reactive antibodies: IgM at 1–3 months, followed by IgG, which persisted in 10–20% and were also found in children born to uninfected mothers. These antibodies were, however, present at lower titers than in infected infants and were apparently produced in response to agent(s) different from HIV. Maternal antibodies toenvdisappeared significantly faster in infected than uninfected infants. Traces of HIV-reactive IgG were present for up to 21 months in children who subsequently seroreverted completely.