Cell volume was measured by wet weight minus dry weight and expressed as ml water per g dry cell solids [ml (g d.c.s.)−1] in RBCs maintained for 60 min under fully oxygenated conditions (Oxy), under continual deoxygenation (≤15 mmHg O2), or under cycles of deoxygenation/re-oxygenation (Oxy/deoxy), in which oxygen tension was changed from 100 mmHg to 0 mmHg every 10 min. Data are means ± s.d., n = 3–7, and in all cases were paired experiments carried out in the absence and presence of 5HMF. #P < 0.05 for control oxygenated RBCs versus control RBCs under the different deoxygenation protocols. *P < 0.05 for deoxygenated RBCs in the absence of 5HMF versus those incubated in its presence. NS, for oxygenated and deoxygenated RBCs in the presence of 5HMF.