Despite the absence of B lymphocytes, peripheral blood lymphocytes (PBL) from four of five patients with congenital agammaglobulinemia (cAγ) generated a specific hemolytic plaque-forming cell (HcPFC) response in vitro to sheep red blood cells and ovalbumin. The kinetics, antigenic, and cellular requirements were similar to normals, but significantly less HcPFC were found in patient cultures. Normal but not patient HcPFC-precursor cells were inactivated by treatment with anti-µ antisera whereas generated HcPFC in both controls and patients were sensitive to treatment with anti-µ. Pokeweed mitogen (PWM) and dextran sulfate (DxS) enhanced the HcPFC-response of normal PBL; cAγ-cells were unresponsive to DxS and, in the presence of PWM, the development of HcPFC was inhibited. These findings indicate the presence of B lymphocyte precursors in the majority of patients with cAγ investigated.