Quasi-elastic light scattering was used to detect the initial stage of aggregation of hemoglobin S solutions. Aggregation of a hemolysate upon deoxygenation was found to occur at concentrations above 11 g/dl, but purified hemoglobin S solutions showed evidence of aggregation only for concentrations greater than 14 g/dl. A transition from a nonaggregated to an aggregated state was observed when the oxygen pressure in solution was reduced below a critical value. The total intensity of the scattered light during the early stages of aggregation was found to increase linearly with time. This result is consistent with a linear association process in which any size aggregate can combine with any other size aggregate.