The protection correlate in case of tetanus toxin is reported in IU/mL, a functional scale referred to an international serum obtained in horses. Recently, we had generated and purified anti-tetanus toxoid Ab by immunoaffinity chromatography. We then quantified the Ab by Lowry method in ug/mL. We had standardized an indirect ELISA for quantification of Ab in mouse and rabbit sera, but the curve is in ug/mL.