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).