The isopropanol precipitation test is widely used for the detection of unstable hemoglobins. A method for accurately and easily determining the amount of hemoglobin precipitated is described. This study demonstrates that changes in incubation time, pH, temperature, and isopropanol and hemoglobin concentrations affect the precipitation of oxy Hb A and oxy Hb S. Hb S shows greater precipitation than Hb A with increasing time, temperature, and isopropanol concentration. Both hemoglobins show equal increases in precipitation, with increases and decreases in pHand decreases in hemoglobin concentration. Comparison of the isopropanol precipitation test with the mechanical shake and heat denaturation tests reveals that the mechanical shake test is the easiest method to study the stability of Hbs A and S. Each of the tests may measure different parameters of stability. The use of all three tests in the evaluation of unstable hemoglobins should be considered.