Newborn BALB/c mice were repeatedly injected either with syngeneic (BALB/c) or xenogeneic (bovine) myosin, albumin, or actin in sterile physiological saline. The serum antibody response was evaluated by enzyme immunoassay 1 and 2 months after birth and after two booster injections. At 1 month, higher antibody titres were found in the sera of mice injected with syngeneic than with xenogeneic antigens. At 2 months and after boosting, anti-syngeneic actin antibodies were present in equal or higher amounts, anti-syngeneic albumin antibodies were not detected, and anti-syngeneic myosin antibodies were considerably decreased. Antibodies produced after booster injections of syngeneic actin were found to be highly specific and to belong mainly to the IgG isotype. These results suggest that newborn mice are better able than adult mice to respond to stimulation with self antigens, and that administration of self proteins during neonatal life may lead to the induction of immunological memory. They also indicate that one of the primary functions of the immune system in newborn mice is the recognition of self antigens.