RBCs were incubated in tonometers at 2% Hct for up to 60 min after which samples were fixed in the samesolution as that used during incubation but with the addition of 0.3% glutaraldehyde. Control experiments showed that this protocol was sufficient to maintain the RBC shape for severalweeks. Sicklingwas assessed by light microscopy. Several hundred RBCs (typically 300–400) were counted using an Improved Neubauer haemocytometer (in five 1mm×1mm squares, the central one and the four corners).