CDC researchers developed a more efficient method of assessing rabies vaccine potency using an antigen-capture electrochemiluminescent (ECL) assay. This assay utilizes SULFO-NHS-Ester labeled murine monoclonal antibodies to quantify glycoprotein concentration, which is an indicator of vaccine potency. Currently, the potency of rabies vaccines is determined by the effective-dose (ED50) mouse study evaluation method, which is more than 50 years old. The labor-intensive ED50 evaluation method has high operating costs, extensive biosafety requirements, and requires the sacrifice of a large number of animals. CDC researchers have addressed these issues by developing a competitive in vitro antigen-capture assay that is rapid, highly robust, reproducible, flexible and much less expensive to implement than the traditional ED50-mouse study evaluation.