The kinetic inhibition of the gelation of hemoglobin S is compared to the change in hemoglobin S soulbility, when the solubility is altered by carbon monoxide, pH, or urea. By means of a new technique, the delay time and the extent of gelation are measured on the same sample. They delay time, td, is found to be proportional to a high power (30-40) of the hemoglobin S solubility. Togehter with the previously reported concentration dependence, this result demonstrates that the rate is proportional to a high power of the supersaturation, S, defined as the ratio of the total hemoglobin S concentration to the equilibrium solubility. The results obey the supersaturation equation td-1 = gammaSn, where gamma is an empirical constant (about 10(-7) sec-1) and n is about 35. The supersaturation equation can successfully account for observations on the kinetics of cell sickling and is therefore used to estimate the increase in the delay time for sickling necessary to produce significant clinical benefit to patients with sickle cell disease.