The responses of lymphocytes to Epstein-Barr virus (EBV) were investigated in five patients with common variable immunodeficiency (CVID). In three patients with CVID in whom the percentages of CD20+ cells and CD21/EBV receptor+ cells were markedly reduced, PBMCs and/or B cells did not respond (or scarcely) to EBV. This may be due to reduction of B cell numbers or reduction of EBV receptor bearing cells. In one patient with CVID in whom the percentages of CD20+ cells and CD21/EBV receptor+ cells were mildly reduced, PBMCs and/or B cells responded well to EBV and secreted immunoglobulin (Ig). This result shows that the patient's B cells with EBV receptors are sufficient to transduct EBV-signals into the nucleus and to respond to EBV. In another patient in whom the percentages of CD20+ cells and CD21/EBV receptor+ cells were not reduced, PBMCs mildly responded to EBV and scarcely secreted Ig. This result shows that the patient's B cells with EBV receptors may not be sufficient to transduct EBV-signals into the nucleus and to respond to EBV.