Abstract Interferon-beta (IFN-β) is an established therapy for relapsing-remitting multiple sclerosis (MS). However, the mode of action and the effect on oligodendrocytes are not yet clear. In this study, we examined the influence of an IFN-β therapy on the proliferation and differentiation of primary oligodendrocyte precursor cells (OPC) in mixed glial cultures. Mixed glial cultures were incubated for 5 days in medium supplemented with 10% of sera from healthy controls, untreated MS patients and IFN-β treated MS patients. Proliferation and differentiation of OPC were determined by immunocytochemistry. Proliferation of OPC was significantly inhibited by sera from untreated MS patients compared to healthy controls, while this effect was almost completely reversed by serum from IFN-β treated MS patients. No effect on OPC differentiation was observed. A prospective and longitudinal analysis of a second cohort of MS patients treated with IFN-β showed that the reversal of inhibition of OPC proliferation was evident after 12 months of treatment but not during the first 6 months. Thus, our results suggest that IFN-β treatment has the capacity to revert the inhibitory effect of serum from MS patients on OPC proliferation. It is currently not clear what this means for regenerative processes.