Lymphoid cells containing cytoplasmic IgM but lacking stable surface IgM are believed to be the direct precursors of B lymphocytes. We have characterized these pre-B cells in the bone marrow of normal individuals and patients with a variety of immunoglobulin deficiencies or hematologic disorders by using immunofluorescence and autoradiography. Pre-B cells comprised 5.8 +/- 5.7% of lymphoid cells in normal bone marrow. Eleven patients with infantile X-linked agammaglobulinemia (X-LA) lacked B lymphocytes but had a normal frequency (3.8 +/- 3.6%) of bone marrow pre-B cells. A smaller proportion of marrow pre-B cells from patients with X-LA were engaged in spontaneous DNA synthesis than was found for normal controls. In individuals other than the group with X-LA, the number of circulating B cells was positively correlated with the frequency of marrow pre-B cells. These results indicate that patients with X-LA have a defect in maturation of pre-B cells, and suggest that some patients with acquired B lymphocyte deficiency may have lost the capacity to generate pre-B cells from stem cells.