IL-10 is a cytokine with suppressive effects on (auto) antigen presentation and T-cell-mediated immune reactions, but is also capable of stimulating polyclonal IgG synthesis. Recent evidence suggests its involvement in multiple sclerosis (MS) and systemic lupus erythematosus (SLE). We assessed PBMNC IL-10 release in MS and SLE patients before and after in vivo prednisone or methylprednisolone treatment. The SLE patients showed a high level of IL-10 release by unstimulated PBMNCs, whereas the PHA-stimulated PBMNCs from MS patients produced large amounts of the cytokine.