A patient with a“silent”mutant hemoglobin characterized by high oxygen affinity and erythrocytosis is described. A novel approach was used to identify the mutant chain. Functionally activeαandβchains were prepared from hemolysates of the patient and a normal control. Hybrid tetramers of patient'sαchain with normalβchain and normalαchain with patient'sβchain were prepared. Functional studies revealed that the patient'sβchains had a higher oxygen affinity (P50. 1.1 torr) than normalβchains (P50, 1.7 torr) and the hybrid containing the patient'sβchains had a P50similar to the patient's“stripped”hemolysate. It was assumed Therefore that the mutation was in theβchain; structural studies using cyanogen bromide cleavage revealed that the patient had Hb San Diego,β109 Val⇒Met, and that the patient's cells contained approximately 50 percent mutant hemoglobin.Functional studies were performed on a. series of mixtures of patient's hemolysate diluted with varying amounts of normal hemo-lysate. Regression analysis of the data extrapolated to an assumed 100% Hb San Diego gave values for P50and Hill's constant of 4.8 torr and 1.0. It is proposed that this method, not previously reported, may be useful in evaluating the functional properties of“silent”mutant hemoglobins that cannot be studied in the purified state.