ABSTRACTThe neonatal period is a state of relative innumodeficiency during which newborns are particularly vulnerable to bacterial, protozoal, and viral infections. In addition, they localize infection poorly, mount a sluggish antibody response to injected antigens and are relatively anergic. Bothin vivoandin vitrostudies suggest that functional immaturity of neonatal macrophages may contribute to this immunologic hyporesponsiveness. Resistance to viral and bacterial infection and production of antibody can be enhanced in neonates by injection of adult macrophages. Specific functional defeats in newborns monocytes have been demonstrated in their capacity to chemotax, to resist intracellular multiplication of virus and to effect antibody-dependent cell-mediated cytotoxicity towards virus-infected target cells. Monocytes from newborns also appear to present antigen poorly, and to co-operate with T-lymphocytes in the suppression of IgG-production by B cells.