Multiple sclerosis (MS) is presumed to be a T-cell mediated chronic inflammatory disease of the central nervous system. We have previously reported that IFN-β_1b (Betaseron) decreases CD3-mediated TNF-α secretion but increases another inflammatory cytokine, IL-6 after three months of treatment. We have now examined cytokine secretion of peripheral blood mononuclear (PMNC) cells after stimulation with OKT3 (anti-CD3) monoclonal antibody (mAb) or Con A in subjects with clinically stable relapsing MS before and three, six and nine months after initiating IFN-β_1b treatment. At nine months Con A-induced TNF-α secretion decreased significantly below baseline but IFN-γ secretion increased above baseline. There were no significant changes in Con A-induced IL-4 over the six month period and no changes in IL-10 and IL-2 over the nine month period. After nine months on treatment the CD3-induced TNF-α and IFN-γ secretion was not significantly different from the original baseline values. Increased CD3-mediated IL-6 secretion in on-treatment compared to pre-treatment samples at three months gradually declined to baseline values by nine months on-treatment. There was no significant changes from baseline compared to nine months on-treatment in CD3-mediated IL-2, IL-4, IL-10. IFN-β_1b (Betaseron) treatment has no clear persistent effect on CD3-induced inflammatory or counterregulatory anti-inflammatory cytokine secretion.