The cellular immune system of the human newborn, like the rest of the immunologic apparatus, is anatomically intact, antigenically inexperienced, and functionally deficient. The latter is suggested by the newborns' enhanced susceptibility to infection, diminished delayed cutaneous hypersensitivity reactions, and selective abnormalities (when compared to adults) of measures of cellular immunity in vitro. These include impaired proliferative response to ubiquitous antigens, depressed lymphotoxin, migration inhibition factor, and immune interferon production, and diminished cytotoxic reactions including cell-mediated lympholysis. By contrast, other aspects of neonatal T cell function, such as to mitogens or allogeneic lymphocytes, natural interferon and leukocyte inhibition factor production, and number and percentage of E-rosette-forming cell are generally normal. These decreased functional properties may provide an explanation for the newborns' susceptibility to infection and for the occasional occurrence of engraftment of foreign cells from either the mother or from prenatal or neonatal blood transfusion.