The oxy-form of sickle hemoglobin (Hb S) is abnormally unstable and precipitates at a 10-fold faster rate than does oxyhemoglobin A (oxy-Hb A) during mechanical shaking. The apparent rate of precipitation of heterozygous hemolysate (AS) is approximately half that of oxy-Hb S. The deoxy-form of Hb S, on the other hand, is resistant to the mechanical treatment. This stabilization is attributed to the conformational change of hemoglobin rather than the lack of oxygen, because carbonmonoxide hemoglobin S, which is known to have conformational properties similar to those of oxy-Hb, is unstable even under anaerobic conditions. Methemoglobin S is most unstable, although addition of cyanide stabilizes the protein. The precipitation of oxy-Hb S is inhibited by ethanol and other organic solvents. The relationship of the mechanical instability of sickle oxyhemoglobin to intraerythrocytic denaturation and vaso-occlusive phenomena in sickle cell disease remains to be determined.Sickle cell disease is due to an abnormal hemoglobin with valine substituted for glutamic acid at the sixth position of the ,-chains. Deoxygenation of erythrocytes results in polymerization of hemoglobin inside the erythrocyte, causing the sickling phenomenon.During the measurement of oxygen equilibrium curves of hemoglobin S (Hb S), we made the serendipitous finding that the oxy-form of Hb S was abnormal in its ease of precipitation. Sickle cell oxyhemoglobin precipitated very quickly with mechanical treatment, including mixing, stirring or shaking (1). This property of oxyhemoglobin S (oxy-Hb S) may have clinical importance because intraerythrocytic precipitation of hemoglobin would cause erythrocyte rigidity and contribute to stasis, increasing the likelihood of prolonged deoxygenation, sickling, and vaso-occlusion. The unusual property of precipitation of oxy-Hb S may be used as the basis of a simple clinical test for the detection of sickle cell hemoglobin in patients with sickling disorders (2).The present paper deals with detailed studies of the nature of the abnormal precipitation of oxy-Hb S in comparison with normal hemoglobin. Shaking Experiments.(1) TCS-shaker (model 150, P. O. Box 141, Southampton, Pa. 18966). Two milliliters of hemoglobin solution (heme concentration, 50-100 MM) were placed in a 15 X 45-mm vial and were shaken with the TCS-shaker for various time intervals at constant temperature. After shaking, the vials were immediately centrifuged at 4000 X g for 5 min and the absorption spectra of the supernatant solutions were recorded between 700 and 480 nm.(2) Vortex mixer (model K-500-4). Three milliliters of hemoglobin solution in a test tube (16 X 150 mm) were vigorously shaken with the Vortex mixer at the medium mixing position. After shaking, the solution was centrifuged at 4000 X g for 5 min. The percent of hemoglobin precipitated is measured by comparing the absorption at 578 nm before and after shaking. When

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