Following culture of human peripheral blood mononuclear cells (PBMNC) from 25 normal donors and 15 patients with common variable immunodeficiencv (CVID), we were unable to identify any IL10-defective patients. Clear-cut effects of IL4 could be demonstrated in controls, while in CVID all effects are less pronounced. While in both controls and CVID baseline levels of 1L6, IgG and IgM were found to be correlated, this was altered by the addition of either IL4 or Poke Weed Mitogen (PWM). We therefore conclude that the inability of PBMNC to produce IL10 is not the cause of CVID in our patients. In CVID, the regulating circuitry triggered by IL4 remains principally intact, however, some subgroups of CVID behave significantly differently.