The effect of HbA and HbF on both the aggregation and deaggregation of deoxy HbS and deoxy HbCHarlem in concentrated phosphate buffers was studied by a turbidimetric method. Although pure deoxy HbA is fully soluble in 2.4 M potassium phosphate buffer, the same concentration of deoxy HbAS was insoluble in the same buffer. Since there was little dissolved hemoglobin in the solute of a mixture of HbA and HbS, and since the intensity of turbidity was approximately twice that which would be expected from deoxy HbS alone, deoxy HbA must have coaggregated with deoxy HbS. Similar studies with mixtures of HbS and HbF showed different results. The rates of aggregation of deoxygenated mixtures of HbS and HbF were much slower than those of similar mixtures of HbS and HbA. Measurements of the absorption spectrum of the solute after centrifugation and the electrophoresis of both the aggregates and the solutes showed that a portion of the HbF was coaggregated with HbS, while some of the HbS was still in solution. The solubility of HbS and mixtures of sickle and nonsickle hemoglobins in 2.2 M phosphate buffer increased in the order of HbS, HbACHarlem, HbAS, HbCHarlem, HbSF, and HbFCHarlem.