Multiple sclerosis (MS) is a demyelinating disorder of the central nervous system of unknown etiology. Immune mechanisms involving the proinflammatory cytokine gamma interferon (IFN-) are believed to play an important role in the pathogenesis of MS. IFN--1b has been introduced as a treatment for MS and was found to reduce the number and severity of clinical exacerbations. To examine the influence of IFN--1b on myelin basic protein (MBP)-specific and phytohemagglutinin-induced IFN- production, we developed a cell-released capturing enzyme-linked immunosorbent assay (CRC-ELISA), which rapidly measures spontaneous and antigen- or mitogen-induced cellular IFN- production. CRC-ELISA documented a significant MBP-specific T-cell response in the blood of untreated MS patients, as assessed by IFN- production. This response was suppressed in MS patients treated with IFN--1b. The present work confirms in vivo the in vitro suppressive effects of IFN--1b on IFN- production in MS. Moreover, it provides a powerful new technique for detection of cytokines.