Common variable hypogammaglobulinemia (CVH) is a clinical syndrome that includes a diverse group of patients with heterogeneous defects resulting in impaired B cell proliferation and terminal differentiation into mature plasma cells capable of normal immunoglobulin synthesis and secretion. In this study, we report our identification of a previously undescribed intrinsic B cell defect in a patient with CVH. This patient's B cells showed a marked impairment in hemolytic plaque-forming cell (HePFC) formation compared with control B cells (15 v 80 HePFCs per culture, respectively). In addition, this patient's B cells displayed decreased B cell colony formation compared with control B cells (5 ± 2 v 93 ± 8, respectively). When examined for their responsiveness to phytohemagglutinin-T cell conditioned media (PHA-TCM), the patient's B cells displayed impaired B cell proliferation compared with control B cells (stimulation index [SI] 1.3 ± 0.20 v 26 ± 1.4 with 20% control PHA-TCM [vol/vol]). Impaired proliferation by the patient's B cells persisted with increasing concentrations of B cell growth factor (BCGF). Additionally, PHA-TCM prepared from the patient's T cells when compared with control PHA-TCM consistently showed less support for control B cell proliferation (SI 1.27 ± 0.21 v 26 ± 1.4, respectively). In coculture studies of B cell proliferation and immunoglobulin synthesis, patient's T cells showed no evidence of an enhanced suppressive effect or decreased helper effect. This patient's immune defects involve, first, an intrinsic B cell defect characterized by an impaired responsiveness to BCGF's proliferation signal and, second, impaired production of BCGF by the patient's T cells.