Responses of common variable immunodeficiency patients' B cells to Staphylococcus aureus Cowan I (SAC) and recombinant interleukin-2 (rIL-2) varied in our study. T-cell function and IL-2 production were normal, and intrinsic B-cell defects were suggested. Some patients showed no increase in expression of IL-2 receptors on B cells with SAC stimulation, which may be due to impaired maturation stages. Two patients showed an increased level of B cells with IL-2 receptors, suggesting blocked development of intracellular mechanisms. One patient may have a defect in immunoglobulin isotype switching.