Treatment of multiple sclerosis (MS) with interferon-beta1b (IFN-beta1b) induces the production of antibodies in some patients. A proportion of these antibodies can reduce the biologic activity of IFN-beta, and they are, therefore, referred to as neutralizing antibodies (NAB). The remaining antibodies do not interfere with the biologic activity of IFN-beta and are referred to as nonneutralizing antibodies (NNAB) in this paper. Immunoglobulin (Ig) subtyping of these antibodies was carried out, and Ig subclass patterns in 20 patients with NAB were compared with those of NNAB in 39 patients. In patients with NAB, IgG2 and IgG4 were found to occur more frequently than in patients with NNAB (30% vs. 3%, p = 0.05, and 55% vs. 18%, p = 0.003, respectively) The NAB titer correlated strongly with the IgG4 titer (r = 0.53, p = 0.02). Median total IgG, IgG1, and IgG4 titers were significantly higher in NAB than in NNAB patients (respectively, 8000 vs. 3200, p = 0.01; 1600 vs. 400, p = 0.0004; 200 vs. 0, p = 0.004). It is concluded that the development of NAB is related to both the quantity and the quality of the antibodies against IFN-beta1b.