A rapid, specific, perfusion immunoassay for active anti-HBsAg monoclonal IgM is described. The immunoassay requires less than 3.5 min per sample. The precision was found to be 3.6% at an IgM concentration of 17 microg/ml. A detection limit of 1 microg/ml IgM in culture media was determined. Assay results were found to correlate very well with standard size exclusion chromatography and radial immunodiffusion techniques. This perfusion immunoassay was demonstrated to be useful for determining anti-HBsAg IgM in complex matrices such as cell culture media. The utility of the immunoassay for monitoring production of anti-HBsAg IgM in a perfusion bioreactor is demonstrated.