Seventy infants born to human immunodeficiency virus type 1 (HIV-1) seropositive mothers were studied for specific antibody (IgA, IgM and IgG) production and the presence of active infection (detectable level of virus in peripheral blood lymphocytes). Among these children, followed for up to 15-40 months after birth, 11 presented unequivocal signs of HIV-1 infection (persistent p24 antigenemia and/or positive virus isolation). Analysis of sera by immunoblotting showed that IgA antibody to HIV-1 p24 core protein, alone or associated with envelope glycoproteins (gp120, gp41), was present in the majority of infected babies (7 of 11), while IgM was found in a lower percentage of cases (4 of 11). No IgA and or IgM antibody to HIV-1 was ever found in babies, born to seropositive mothers, who seroreverted after birth or in the control group enrolled in this study. Our results indicate that immunoblotting analysis of IgA antibody to HIV-1 polypeptides may represent a useful complementary prognostic marker in children born to HIV-1 seropositive mothers.