The transplantation immunity of viviparity has been studied by orthotopic transplantation of skin between outbred mothers and offspring of four mammalian species. Specifically delayed rejection, evanescent and often mutual to mother and offspring, was demonstrated during and after pregnancy in armadillos, rats, dogs and sheep. Delay of rejection was most clear in the armadillo and the rat and least marked in the dog and the sheep. The term 'immunological inertia of viviparity' has been applied to this phenomenon because special features distinguish it from other states of diminished immunological reactivity. The causative mechanisms are unknown but the phenomenon is believed to be one type of specific adaptive immunity.