Proliferation of blood leucocytes in cord blood and during the first 12 days of life was studied by autoradiography after a brief in vitro labelling with 3H-thymidine. Large numbers of labelled erythroid precursors were present in cord blood. During the first 3 days of life, the majority of labelled cells were immature myeloid cells. The percentage of labelled lymphoid cells rose more than tenfold 4-7 days after birth without any concomitant increase in the absolute numbers of blood lymphocytes. Most of the labelled lymphoblasts were able to form rosettes with sheep erythrocytes. Such a physiological neonatal lymphoblast response would appear to be a manifestation of cellular rather than humoral immunity and probably represents an immunological response to the antigenic challenge encountered after intrauterine life.