The present invention relates to an immunoassay for simultaneously measuring the presence of antibodies to a plurality of HPV types that utilizes particle-based flow cytometric analysis. The presence and/or titre of neutralizing antibodies in a test sample are determined in a competitive format, where known, type-specific, fluorescently labeled neutralizing monoclonal antibodies compete with antibodies within a test sample for binding to conformationally sensitive, neutralizing epitopes on specific HPV-VLPs. The invention also provides a microsphere complex comprising a microsphere coupled to an HPV VLP.