ABSTRACT: A colony of fully pedigreed Monodelphis domestica has been used to investigate the maternal-fetal relationship in this unique marsupial species. To determine how immunity is transferred from mothers to young in M. domestica, we hyperimmunized females with sheep red blood cells (SRBC) before and during gestation. Offspring from these females were collected at various times after birth, and saline extracts of the neonates were assayed for hemolysins against SRBC. Antibodies were present in extracts of newborn that had been allowed to suckle their mothers; none were detected in extracts of infants that were not allowed to suckle. Antibodies were present in the milk of immunized mothers, but were not detected in the milk of nonimmunized mothers. The titer of antibodies in the extracts of newborns generally increased proportionately to the time that the newborn had been allowed to suckle. We conclude that the transfer of passive immunity from mothers of M. domestica to their offspring occurs primarily via the milk.