Interferon-gamma is produced by immune cells before MS exacerbations, and exogenous IFN-gamma treatment causes MS attacks. IFN-beta production, conversely, rises after exacerbations. IFN-beta therapy ameliorates MS, possibly by lowering IFN-gamma secretion and inhibiting responses to IFN-gamma. IFN-gamma effects are regulated by IFN-gamma receptor (IFNGR) expression. IFN-gamma is pro-inflammatory at low IFNGR levels, but induces apoptosis in cells with high IFNGR levels. We studied effects of IFN-beta1a therapy on IFNGR expression on PMA/ionomycin-stimulated PBMNC's in 29 patients with active and stable MS. Surface IFNGR-alpha (the binding chain) and IFNGR-beta (signaling chain), as well as intracellular IFN-gamma and IL-10, were measured with flow cytometry. Before IFN-beta therapy, intracellular IL-10 was depressed and the IFN-gamma/IL-10 ratio was elevated in MS, particularly during clinical activity. With IFN-beta therapy IL-10 levels increased, suggesting that a Th2 deficit was reversed. The IFNGR-alpha chain was significantly elevated on lymphocytes in stable and active MS patients not receiving IFN-beta therapy. Expression of the IFNGR-beta chain was low during active untreated disease. After IFN-beta therapy, the IFNGR-beta/alpha ratio increased at 3 months and fell at 12 months. Increased susceptibility to apoptosis with high IFNGR-beta chain expression at 3 months is likely to remove activated T cells during IFN-beta therapy.