Systemic Lupus Erythematosus (SLE) is a multisystem disease characterized by an increase in the secretion of autoantibodies. The mechanisms of autoantibody-induced disease have not been clarified. 1,25-Dihydroxy-vitamin D3 (1,25-D3) is known to be important in the regulation of normal human lymphocyte functions. Among its regulatory functions is the ability to inhibit mitogen-stimulated production of immunoglobulin. The experiments reported here explored the regulation of IgG production by peripheral blood mononuclear cells (PBMCs) of patients with inactive and active SLE. 1,25-Dihydroxyvitamin D3 inhibited mitogen-stimulated IgG production in cells from normal individuals and inactive SLE patients, but not spontaneous IgG production by PBMCs from active SLE patients. Addition of exogenous IL-2 (5-50 U/ml) to 1,25-D3-treated cells from all patient groups did not affect IgG production significantly under any conditions tested. The addition of IL-2 to PMBCs had no effect on IgG production in normal individuals or inactive SLE patients, but stimulated IgG production in PBMCs of active SLE patients. We conclude that the regulation of mitogen-stimulated IgG production in inactive SLE patients by 1,25-D3 and IL-2 is similar to normal individuals, but IgG production by active SLE PBMCs is unresponsive to 1,25-D3 regulation and is increased with the addition of IL-2.