The interaction of hemoglobin C (Hb C) with erythrocyte membranes was studied using changes in fluorescence intensity in a membrane-embedded probe. The affinity of Hb C for the membranes at pH 6.0 and pH 6.8 was compared to that of normal hemoglobin (Hb A). Steady-state and kinetic data were delivered. The affinity of Hb C for the erythrocyte membrane at pH 6.8 appeared to be about five times greater than that of Hb A. The associations of Hb C and Hb A with the membrane were reversible to about the same extent. The cytoplasmic portions of band 3 membrane proteins were suggested to be the binding sites for both hemoglobins. The membrane binding of Hb C at pH values of 6.8 to 7.0 indicates that this reaction may occur under physiological circumstances.