Therapeutic proteins like recombinant human interferon-beta (rhIFNbeta) may induce neutralising antibodies (NABs), which inhibit their efficacy. Hence, there is a great need for strategies to predict whether a formulation will induce an immune response. Immune tolerant transgenic animals are important tools to study this phenomenon. This article describes a bioassay for NABs detection in mouse sera. The bioassay corresponds to the MxA Gene expression Assay (MGA) for human sera and measures the inhibition by mouse serum of the IFNbeta induced MxA mRNA. Samples from 6 non-immunised and 14 IFNbeta-immunised mice were tested for both binding antibodies (BABs) and NABs using the bioassay. All 16 mouse sera tested positive for NABs were also positive for BABs; BAB and NAB levels were correlated with a coefficient of 0.62 (p=0.0186). The intra-assay variations ranged from 1.38% to 5.26% (mean 3.03%). Effects of cytotoxicity against A549 cells were slightly evident at low serum dilutions (i.e. 1/10, 1/20), but levels of damaged cells were easily evaluated based on the threshold cycle (Ct) values of the housekeeping gene 18SrRNA. The possibility of measuring NABs, in addition to BABs, in mouse sera increases the usefulness of the animal model, in studying the many factors influencing the immunogenicity of rhIFNbeta.