In vitro immunoglobulin and IL-6 synthesis were investigated in patients with CVID and healthy controls. The IL-6 production was increased in CVID patients. However, after calibration to equal monocyte counts in the mononuclear cell population the difference between patients and controls was no longer significant. The immunoglobulin production in CVID was decreased but no correlation between immunoglobulin and IL-6 production was found. From these results we suggest that monocyte activation might be of importance in terms of secondary organ lesions frequently occurring in CVID patients.