Hemoglobin A was modified in vitro with 0.02—0.03 M glutarylsalicylamide for two hours at pH 7.2 and 37°C. The extent of modification was about 30—50%, as estimated by visual comparison after electrophoretic separation. A substantial decrease in oxygen affinity of modified hemoglobin solutions was observed. Similar results were also obtained for dilute cell suspensions of washed red blood cells and whole blood after GSM modification. Other properties such as cooperativity, Bohr effect and 2,3-DPG dependence remained essentially unchanged. Although the site(s) of modification have not been determined, it is unlikely that they would involve any amino acid residue contributing to the above allosteric properties.