There are conflicting reports concerning changes in vascular reactivity following interactions between erythrocytes and vascular endothelial cells (Nath et al., 2000; Olmos et al., 2002). Hemoglobin(Hb) has been described as a scavenger of Nitric oxide. The goal of this study was to characterize the effect of constituents of erythrocytes from subjects with different Hb genotypes on acetylcholine dependent vaso-relaxation. Isometric contractions of ring preparations (5mm long) of rabbit carotid artery (n=6) suspended in 20ml organ baths and bubbled with 95% O2, 5% CO2 were studied pharmacologically using standard in vitro techniques under an initial load of 1g, at 37oC and pH 7.4. Concentration-dependent contractile responses induced by phenylephrine (PE) as well as relaxation responses induced by acetylcholine (Ach) were examined and their respective EC70 and IC70 values obtained. Relaxation responses of pre-contracted control tissues as well as in tissues exposed for 30 minutes to (a) intact washed erythrocytes (b) erythrocyte ghosts and (c) haemoglobin – all obtained from subjects of different haemoglobin genotypes (AA, AS and SS) were recorded. Arterial rings were exposed to 50µl of each of the erythrocyte constituents at an adjusted haematocrit of 0.6. Ach-induced relaxation was significantly enhanced (p<0.05) by AA erythrocytes (46.2±3.25 and 69.5±5.4% for control and test, respectively). In contrast, AS and SS erythrocytes as well as the Hemoglobin solution did not significantly alter Ach relaxation whereas AS and SS ghosts significantly (p<0.05) attenuated Ach relaxation. Compared with AA, intact erythrocytes from AS and SS subjects elicited significantly greater inhibition of Ach relaxation; furthermore, inhibition of Ach relaxations by erythrocyte ghosts were significantly greater with AS than SS. We conclude that a membrane-bound factor may account for the genotype-dependent attenuation of Ach-induced relaxation following interaction between erythrocytes and vascular endothelial cells.