Blood specimens from 89 human embryos and fetuses were analyzed by immunological methods during gestation weeks 4–40. All formed elements circulate in fetal blood starting from week 13 of gestation. Lymphocytes predominate among blood leukocytes, because normally lymphocytosis is an inherent, feature of blood. T cells and their subpopulations: active T lymphocytes, Tγ lymphocytes, and theophylline-sensitive and risistant, cells are present. B lymphocytes are represented by cells with immunoglobulins M and G, zero cells, and large granular lymphocytes.