Measurement of the proportion of abnormal hemoglobin in a hemolysate is essential for differentiation of simple and compound heterozygotes (e.g., sickle cell trait vs. sickle thalassemia), for differential of alpha- and beta-thalassemia in compound heterozygotes, and for differentiation of various types of beta-thalassemia in such persons. Utilization of such measurements is hampered by the imprecision and inconvenience of current methods. We have adapted a readily available agar electrophoresis method for this purpose, scanning unstained gels at 420 nm. The new method is particularly valuable for rapid estimation of percent of HbS after partial exchange transfusion in patients with sickle cell anemia. It cannot be used for quantitation of HbF or for quantitation of hemoglobins that comigrate with HbA; contrariwise, it can be used for hemoglobins that only separate from HbA on agar (e.g., HbBethesda).