were then tested on the main cation pathways which mediate solute loss and dehydration of RBCs from SCD patients, under fully oxygenated and fully deoxygenated conditions. Results are shown in Fig. 2 for RBCs fromhomozygous (HbSS) patients. In the presence of o-vanillin, KCC in oxygenated RBCs was substantially inhibited (by about 75%). Pre-treatment with o-vanillin for 30 min prior to flux measurement produced a slight increase in inhibition. In these RBCs, KCC activity was reduced by about half by deoxygenation and this residual oxygen-insensitive component of KCC was also sensitive to ovanillin (inhibition of this component of KCC activity was 73±13% without pre-treatment,means± S.E.M., n=5). For Psickle activity, the effect of o-vanillin in the absence of pre-treatment was insignificant. Following pre-incubation with o-vanillin, however, Psickle activity was inhibited by about 50% (Fig. 2).