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The clinical investigation of murine monoclonal antibodies (MoAbs) as potential immunotherapeutic agents necessitates their quantification in human serum. The present paper reports the development of a sensitive, non-competitive “sandwich” immunoradiometric assay specifically optimized for measurement of murine IgG in human serum. Affinity-purified goat anti-mouse IgG antibody covalently bound to acrylic microspheres serves as the solid-phase antibody and ¹²⁵I-labelled goat anti-mouse IgG antibody functions as tracer. All assay reagents were obtained from commercial sources. The assay is sensitive (capable of detecting 20 ng ml⁻¹ of murine IgG), specific (< 0.0001% cross-reactivity with human IgG), reproducible (intra-assay %RSD < 6.3%), and rapid (a 100-tube assay can routinely be processed in 3 h). The assay has a working concentration range of 20–2000 ng ml⁻¹ and is suitable for measuring …