Abstract
Summary: Infant rats born of mothers sensitized prior to and during pregnancy displayed a) a state of delayed hypersensitivity as manifested by skin reactions to tuberculin in 26% of animals born of mothers immunized with tubercle bacilli and b) a state of homograft immunity manifested by accelerated rejection of donor homografts in infants born of homografted mothers. Participation of maternal cells in the rejection of homografts in infants was excluded in an experiment in which highly tolerant mothers were used. Accelerated rejection was observed in infants of these mothers after the latter had been injected with trypsinized skin cells. The most likely basis for the accelerated graft rejection appeared to be a process of active immunization resulting from passage of antigen across the placenta.