Monoclonal antibodies (mAbs) to normal human hemoglobins (Hbs) A and F and to variant Hbs C and G-Philadelphia were conjugated to horseradish peroxidase (HRP) and used in qualitative or quantitative enzymelinked immunosorbent assays (ELISAs). Conjugates with output molar HRPilgG ratios close to 2.0 had higher avidity for the cognate antigens than those with ratios above or below 2.0. The analytical sensitivities of the conjugates ranged from 0.2 to 4 ng of hemolysate containing the target hemoglobin, and it was not related to the input or the output HRPilgG ratios. The overall imprecision for the qualitative ELISA was below 8%, and the accuracy for the identification of Hbs C and G-Philadelphia was 100Β°/~ as compared with established methods. Quantitative determinations of HbA based upon direct dose-response curves showed an analytical sensitivity of 1% and an imprecision G 11%. The most significant application of the HbA assay was in the differential diagnosis of hemoglobinopathies associated with partial or total suppression of HbA synthesis. Corn- petitive dose-response curves for the HRPI anti-y conjugate allowed the quantification of HbF in the clinically significant range of 0.5-1 O%, with an imprecision s 12%. It is concluded that the incorporation of HRPimAb conjugates into the ELISA technique offers a simpler, more rapid, yet specific alternative for the measurement of hemoglobins.