X-linked agammaglobulinemia (XLA) has been described as a disorder in which pre-B cells fail to differentiate into B cells. However, a small number of B cells have been seen occasionally in patients with this disorder. Because the phenotype of these cells might be helpful in defining the site of the defect in XLA, immunofluorescent staining techniques were used to characterize the B cells that can be found in patients with XLA. Surface IgM-positive B cells could be detected in the peripheral circulation of all seven patients studied. These B cells constituted a very small percentage of the total lymphocytes (0.01 to 0.3% compared with 3.2 to 13.7% in controls) and differed in phenotype from control B cells. They were much more brightly stained for surface IgM (p less than 0.001) and less brightly stained for Ia (p less than 0.01). This phenotype is similar to that described for immature B cells in the mouse. Over 80% of the patients' B cells expressed surface IgD, and all expressed the B cell marker B1, but only 35% expressed the B cell marker B2. This B cell marker, which is the C3d receptor and the Epstein-Barr virus receptor, is expressed later in ontogeny than B1 and can be detected on over 80% of control B cells. All B cells expressed either kappa or lambda light chain. These findings indicate that the defect in differentiation of pre-B cells into B cells is not absolute in patients with XLA. The immature phenotype of the B cells additionally suggests that there may be a block in the maturation of B cells at more than one stage of differentiation in this disorder.