Oxyhemoglobin S exhibits greater mechanical instability than oxyhemoglobin A. The rate of precipitation of Hb S when agitated by vortexing depends upon the geometry of the tube, the volume of the hemoglobin solution, and the concentration of hemoglobin. The rate of precipitation is inversely related to concentration. Precipitation is inhibited by temperatures near 4 degrees C and alkylureas whose protective capacity is approximately proportional to the carbon chain length of the alkyl group. Blocking the beta93 -SH group with parahydroxymercuribenzoate has only a small enhancing effect on the precipitation rate. Other mutants such as Hb Gun Hill, Leiden, (both heat unstable), and C-HARLEM are also unstable. In the case of C- HARLEM, the precipitation rate is greater than that for Hb S. The heat- unstable mutants are not as well protected by cold temperatures or alkyl ureas. D2O has only a minor stabilizing effect on hemoglobin S, but NaCl and related salts markedly enhance precipitation at concentrations of 0.5 M. It is concluded that mechanical instability of oxyhemoglobins is a multifactorial process involving surface denaturation, pH, ionic strength, hydrophobic interactions, protein conformation, and primary protein structure. This phenomenon will require more extensive investigation.