Density changes that occur in T⁺ sickle cells during short oxygenated incubations and their dependence on HbF. Fraction II cells were treated with nystatin to normalize their K⁺ content and hydration and incubated for 20 minutes at pH 7.2 in the presence and absence of chloride. A secondary density fractionation was performed and the T⁺ cells were identified. (A) The secondary density distributions of all RBCs, T⁻ cells, and T⁺ cells before nystatin treatment, after nystatin treatment, and after incubation of the normalized cells in the presence or absence of chloride. The expected chloride-dependent density shift in T⁺ cells takes place. (B) For the postnystatin and chloride-incubated samples, cells were also analyzed for HbF, and the density distributions of T⁺F⁻ and T⁺F⁺ cells were determined. Both subtypes had the same tendency to become more dense when incubated under these conditions.