Patients with active SLE often have an ongoing production of IFN-α. We therefore searched for an endogenous IFN-α-inducing factor (IIF) in SLE patients and found that their sera frequently induced production of IFN-α in cultures of peripheral blood mononuclear cells (PBMC) from healthy blood donors, especially when the PBMC were costimulated with the cytokines IFN-α2b and granulocyte-macrophage colony-stimulating factor (GM-CSF). The phenotype of the IFN-α-producing cells (IPC) as determined by flow cytometry corresponded to that of the natural IPC, resembling immature dendritic cells. The IIF activity in SLE sera was sometimes as high as that of a virus and was present especially in patients with active disease and with measurable IFN-α levels in serum. The IIF had an apparent molecular weight of 300–1000 kD and appeared to consist of both immunoglobulin and DNA, possibly being immune complexes. This endogenous IFN-α inducer may be of pathogenic significance, since a reported occasional adverse effect of IFN-α therapy in patients with non-autoimmune disorders is development of anti-dsDNA antibodies and SLE.