The nature of lymphoid progenitors and factor(s) determining commitment to either the T- or B-lymphocyte pathway are poorly understood in the human system. In this study, we generated a monoclonal antibody (MoAb), 18.6, that recognizes a cell surface antigen on a human lymphoid progenitor cell line (FL4.4). MoAb 18.6 reacted with lymphoid progenitor lines, B lymphoid cell lines, and myelomonocytic cell lines. It did not react with any T cell or erythroid leukemic cell lines. Two color FACS analyses of normal lymphoid tissues showed that MoAb 18.6 reacted with a majority of CD20+ mature B cells and a minority of CD64+ monocytes. Molecules of 3 different sizes with MW of 34, 45, and 68 Kd were precipitated with MoAb 18.6 from the lymphoid progenitor cell line. The 18.6 antigen was not expressed on a fetal liver-derived lymphoid progenitor-like cell line, FL1.4, which has the capacity to differentiate into microglia-shaped cells upon PMA-stimulation. Stimulation of FL1.4 cells with PMA induced expression of the 18.6 antigen within 24 hr and the microglia-shaped cells stained positively with MoAb 18.6. Finally, cloning of a cDNA that encoded the 18.6 antigen revealed that the 18.6 antigen is identical to the CD23 antigen. Taken together, these data suggest that the 18.6/CD23 antigen is expressed on lymphoid precursors at a very early stage of differentiation.