One neutralization unit is the amount of antiserum required to neutralize one unit ofhuman interferon beta (Hu-IFN-beta ) to a 50% endpoint. Interferon was titrated with the use of the cytopathic effect inhibitionassay as described [Rubinstein, S., Familletti, P.C., and Pestka, S. (1981) "Convenient Assay for Interferons," J. Virol. 37,755-758; Familletti, P.C., Rubinstein, S., and Pestka, S. (1981) "A Convenient and Rapid Cytopathic Effect InhibitionAssay for Interferon," in Methods in Enzymology, Vol. 78 (S. Pestka, ed.), Academic Press, New York, 387-394]. In thisantiviral assay for interferon about 1 unit/ml of interferon is the quantity necessary to produce a cytopathic effect of 50%.The units are determined with respect to the international reference standard for Hu-IFN-beta provided by the NationalInstitutes of Health [see Pestka, S. (1986) "Interferon Standards and General Abbreviations," in Methods in Enzymology(S. Pestka, ed.), Academic Press, New York 119, 14-23]. This material is prepared specifically for effective neutralizationof Hu-IFN-beta.