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The immunogenesis of the human fetus has been investigated by means of the formation of immunoglobulins in vitro, immunofluorescence, morphological studies, and analysis of the immunoglobulins in the serum. Twenty fetuses which were born alive but died soon after delivery, were studied; their ages ranged from 13 to 31 weeks. The results of the spleen cultures demonstrated the synthesis of IgG and IgM, which starts at about the twentieth week of gestation. In the serum, IgM could be detected at about the same period. The immunofluorescent staining of the spleen tissue showed that medium sized and large lymphoid cells as well as plasma cells, even with Russell bodies, were positive for either IgG or IgM. The peripheral blood was also found to contain a small number of medium sized IgG and IgM-positive cells. Both the spleen and the peripheral blood showed a considerable number of fluorescent small lymphocytes which exclusively contained IgM. The relatively high ratio of IgM to IgG production prenatally as compared to the postnatal situation, agrees with a predominantly primary antibody response in fetal life. In general, the fetal thymus did not synthesize immunoglobulins. No indications for the synthesis of IgA and IgD during fetal life were found.