Cells were stored overnight in CPDa anticoagulant and centrifuged in Wintrobe tubes at 2000g for 30 minutes at 20°C. The packed cells were then separated by aspiration into 4 fractions (no. 1 uppermost) after which the cells were reconstituted in plasma to original hematocrit. Red cell indices were measured in a Sysmex SE900 counter (Milton Keynes, United Kingdom) and intracellular [Na⁺] and [K⁺] were determined on washed cells by flame photometry. Indices on whole blood (ie, prior to centrifugation) are shown for comparison. Effective separation by density was achieved, as evidenced by the lower MCHC in the upper fractions, but while the reticulocytes were enriched in the upper fraction in normal blood (0.018 in the uppermost fraction compared with 0.002 in the lowest), the reticulocytes were enriched in the lower fraction in both patients (0.133 and 0.26 in lower fractions compared with 0.077 and 0.19 in uppermost). The specimens used here were stored overnight. In some leaky red cell conditions, notably cryohydrocytosis, very marked changes in red cell electrolytes can occur on cold storage, but neither pedigree here shows such an effect.