Diphtheria antitoxin antibodies were measured by solid-phase enzyme-linked immunosorbent assay (ELISA) on the basis of a calibrated antibody standard. Protective antibodies were expressed as International Units per ml (IU/ml). The method of quantitation employed is easily performed under routine conditions. Sensitivity proved to be 0.09 mIU/ml, and direct measurement of antitoxin concentrations ranging from 0.12 to 8.30 mIU/ml was carried out by using the standard curve. With a routine assay dilution of 1:100, serum concentrations from 12 to 830 mIU/ml were covered.