The deformability of erythrocytes from patients with sickle-cell anaemia was measured with a St George's blood filtrometer at a range of oxygen partial pressures and at four levels of zinc loading. 2. When incubated in buffered saline containing zinc and the chelator ethyl maltol, erythrocytes rapidly accumulated zinc and thus their oxygen affinity was increased. 3. Neither the oxygen partial pressure nor zinc loading affected the filtration of normal erythrocytes. 4. Deoxygenation of sickle erythrocytes greatly impaired filtration, although the initial filtration rate declined sharply at different oxygen partial pressures (between 70 and 35 mmHg) in different patients. 5. Low levels of zinc (0.03 +/- 0.003 mol of zinc/mol of haemoglobin tetramer) were without effect on sickle cells, but at zinc/haemoglobin ratios of 0.6:1 and above, the sharp fall in filtration rate occurred at oxygen partial pressures 8-25 mmHg below the oxygen partial pressure that impaired filtration of untreated cells. 6. Hence, the deformability of sickle erythrocytes in vitro can be improved by increasing the intracellular content of zinc to 20-fold above normal. Further studies are now required to examine the stability of zinc in erythrocytes, the effects of high intracellular zinc concentrations on erythrocyte viability, and the toxicity of zinc released from zinc-laden cells.
