Considerable information is now beginning to accumulate on the ontogeny of immune responses in the ovine fetus. The fetal lamb has served as an important model for much of our present knowledge. Many critical questions remain to be answered in the ruminants. Much of this information provides a basis for a better understanding of the susceptibility of the fetus, the character of lesions and the events leading to persistent infections in congenital infections. The role that hormones associated with pregnancy plays in active and passive immunity in maternal, fetal and neonatal systems needs further definition. An understanding of these may make it possible to assist in reducing infections leading to congenital or neonatal disease.