A high proportion of carriers of classic hemophilia can be identified in the laboratory because, in comparison to normal women, the concentration of antigens related to antihemophilic factor (AHF, factor VIII) that are detected in their plasma by heterologous antiserum (factor VIIIR:Ag) is relatively higher than the titer of AHF that is measured in clotting assays (factor VIII:C). Enzyme-linked immunosorbent assay (ELISA) appears to overcome some of the technical difficulties associated with measurement of AHF-like antigens. The results of ELISA correlated closely with those obtained by semiquantitative immunoelectrophoresis, except in patients with von Willebrand's disease. In which ELISA appeared to provide a more quantitative estimate of AHF-like antigen. Utilizing the ELISA technique and a revised method of logarithmic discriminant analysis, we were able to distinguish all of 37 obligate carriers of hemophilia at the level of certainty that would have misclassified 5% of normal women as carriers. The relative simplicity of ELISA suggests its utility in the diagnosis of the carrier state in the female relatives of hemophiliacs.