The immune response of the neonate is poor and is dependent on passive immunity provided by maternal Ig. However, here we show that exposure of the neonate to environmental antigens induces a germinal center (GC) reaction. In the peripheral blood of premature infants one finds IgG class switched B cells expressing a selected V-gene repertoire. These data suggest that restrictions in the repertoire rather than immaturity of the immune system is responsible for the poor immune responses of the neonate.