In the first series of experiments, the effect of o-vanillin (5mM) was tested on sickling of RBCs fromHbSS patients (Fig. 1). In fully deoxygenated RBCs, therewas only a small reduction in percentage sickling (N.S.) in the presence of o-vanillin. At higher O2 tensions, nearer the P50 for O2 saturation of Hb, greater effects were observed, however, so that at an O2 tension of 15 mm Hg, sickling was inhibited by about 75% in the presence of o-vanillin (Fig. 1).