To examine the influence of maternal lymphocyte factors on the immune responses in offspring in early life, antibody production in neonates born to either normal or lymphocyte-deficient mothers was analyzed. Recombination activating gene (Rag)-2 + / - mouse neonates born to Rag-2 + / + , Rag-2 + / - or Rag-2 - / - mothers were injected with goat anti-mouse IgD antiserum, and IgE and IgG 1 production was evaluated. The levels of IgE and IgG 1 were higher in the pups born to Rag-2 + / + and Rag-2 + / - dams than to lymphocyte-deficient Rag-2 - / - dams. The enhanced antibody production in the former compared with the latter neonates was also found following immunization with ovalbumin or TNP-Ficoll. Thus, the presence of maternal lymphocyte factors was suggested in neonates that augmented antigen-specific antibody production in both T cell-dependent and -independent pathways. A reduction in antibody production was observed in normal neonates when they were foster-nursed by Rag-2 - / - mothers. Thus, the maternal lymphocyte factors enhancing the immune responses in newborns were shown to be present in breast-milk.