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Morphological characteristics of B cell in human embryonic and fetal livers during the first trimester of gestation were examined. Light microscopically, CD9+, CD10+, CD19+, and CD20+ cells of B cell lineage became detectable as small lymphoid cells from 8 weeks gestation. Tdt+ cells first appeared also as small lymphoid cells on the 43th day of gestation. Ia+ or CD34+ cells in embryonic livers between the 33th and 43th day of gestation were large blastic cells resembling myeloblasts while some of Ia+ or CD34+ cells after the 43th day of gestation as well as Tdt+ cells were similar to lymphocytes. Electron-microscopically, all Ia+, CD10+, and CD19+ cells existed solitarily in intercellular spaces of hepatocytes, but not in intravascular spaces. Ultrastructural aspects of these cells were distinguishable each other. These findings indicate that 1) B cells developed and differentiated in the fetal liver, but the fetal liver during the first trimester was not a lymphoid organ, 2) lymphohemopoietic progenitor cells were derived from Tdt+ cells in the livers between 43th and 56th day of gestation. Ia+ cells detected as a small lymphoid cell in the liver at the 50th day were considered to be progenitor cells of lymphocytic lineage.