SummaryBetween 1:120 and 1:180 of human newborn T cells proliferate in limiting dilution cultures with allogeneic lymphocytes or with la-bearing monocytic stimulator cells. The proliferating responder cells were derived from both the OKT 4+and OKT 8+subsets as determined by immunofluorescence and by thymidine uptake. Five to seven days after an exchange blood transfusion there was a slight increase in the percentage of OKT 8+T lymphocytes in the recipient's blood. Newborn blood also contains a population of non-T cells which proliferate in the absence of allogeneic stimulator cells. In limiting dilution cultures, the frequency of these spontaneously dividing cells was 1:3125 of mononuclear cells. Our results suggest that the newborn T lymphocyte proliferative response to alloantigen is mature by the time of birth and they provide no phenotypic explantion for the previous report of mixed lymphocyte culture-induced suppression by newborn T cells. The predominance of newborn metaphases in 2-way mixed lymphocyte cultures with adult cells (on which the previous report of suppression was based) is not seen if the non-T (stimulator) cells are irradiated. These results suggest that the data previously interpreted as evidence for suppression arose through proliferation of newborn non-T cells.