Neutralizing antibodies (NABs) against IFN beta should be measured in specialized laboratories, using a test of inhibition of the cytopathic effect (bioassay or CPE test), based on the capacity of IFNss to block the infection of live monolayer-cultured cells by a virus, depending on the presence or absence of NABs. The European Federation of Neurological Societies (EFNS) considers this assay to be the gold standard. However, the various different ways to perform this assay complicate comparison of the results between laboratories. The World Health Organization (WHO) has published several recommendations to perform this assay using the A549 cell line and the murine encephalomyocarditis virus (EMCV). In order to validate the results previously obtained in our laboratory with HEP2/VSV, we undertook a comparative analysis of the two bioassays, HEP2/VSV and A549/EMCV, to assess whether the use of different cell lines and viruses influences sensitivity. We also calibrated the A549/EMCV assay with a reference IFNss. Our results confirm that the bioassay with HEP2/VSV is as sensitive as the assay with A549/EMCV and that a significant association and correlation exist in the results between both assays. Thus, past results with HEP2/VSV in our laboratory could be comparable with those obtained with A549/EMCV in both our laboratory and others.