In vitro, two different classes of antibodies are recognized according to the assay used to identify them: binding antibodies (BAbs) and neutralizing antibodies (NAbs). There is emerging agreement that NAbs are correlated with reduced therapeutic efficacy of IFNβ. None of the pivotal trials in relapsing-remitting multiple sclerosis (MS) showed an effect of NAbs on disease progression. Among the therapeutic monoclonal antibodies natalizumab, alemtuzumab, and daclizumab are humanized monoclonal antibodies, whereas rituximab is a mouse-human chimeric antibody. NAbs are associated with reduction of the therapeutic effect of IFNβ, and patients with persistent high titers of NAbs should be switched to an alternative therapy. Patients with low or intermediate titers should have their IFN bioactivity measured with an in vivo mRNA MxA induction test. Patients with repeated abrogation of the mRNA MxA response to IFN injections should be switched to another therapy.