Abstract
Female CBA mice that had been thymectomized soon after birth were mated with normal male T6 mice. After delivery of at least one litter, the parous females exhibited normal immune reactions to sheep erythrocytes and skin homografts. The transplacental passage of a humoral substance from the thymus glands of the developing fetuses is suggested as the mechanism responsible for the restoration of immunological responsiveness in neonatally thymectomized parous female mice.