The aim of this study was to investigate the influence of neutralizing antibodies in mother's serum on the risk of mother-to-child transmission of human immunodeficiency virus type 1 (HIV-I). Sera from 20 HIV-I infected mothers were analyzed for their ability to neutralize their own virus (autologous neutralization) and virus obtained from other mothers (heterologous neutralization). A statistically significant correlation was found between the capacity to neutralize 1 selected primary isolate and protection of the child from infection. Also, neutralizing antibodies against autologous virus were more frequently present in nontransmitting mothers than in transmitting mothers (5 and 2, respectively, of 10 mothers). The mothers with autologous neutralizing antibodies also neutralized at least 2 heterologous primary isolates. Thus, mothers with neutralizing antibodies to primary HIV-1 isolates have a reduced risk of infecting their children.