The degree of activation of B and T cells in the developing spleen during postnatal life was studied in germ-free (GF) and specific-pathogen-free (SPF) BALB/c mice of the same breeding stock. We found that the progeny of GF mothers up to 3 weeks of age contain higher numbers of activated splenic cells than baby SPF mice, thus suggesting qualitative differences in maternally-derived antibodies. This "advantage" of GF mice is also indicated by an anticipated maturation of the splenic lymphoid compartment and is reflected in higher frequencies of B and T lymphocytes in adult life. In both kinds of mice, the frequency of activated cells is very high at birth and then declines, reaching minimal values by 4 weeks of age. Later, activated B cells increase sharply in SPF mice, suggesting polyclonal activation mediated by bacterial products. Results are discussed on the basis of the mutual influences between B and T cells in the establishment of a functional network.