The fluidity, defined by its two components, the order parameter, S, and the rotation correlation time, tau c, was studied on healthy human erythrocytes ghosts. We also measured ghost protein, cholesterol and phospholipid contents as well as acetylcholinesterase activities. No statistically significant difference was evidenced between erythrocyte ghosts from men and women. Whereas tau c values did not significantly vary among sample elements, variations of ghost order parameters about the mean were explained at 61% by changes in cholesterol contents and, to a lesser extent, in protein contents. No relationship was evidenced between ghost order parameter values and those of corresponding acetylcholinesterase activities. Liposomes prepared from ghost lipid extracts had much lower order parameter values than did corresponding ghosts. A few experiments were performed in the same way on ghosts from sickle blood. This disease appeared to decrease the bilayer lipid motionnal freedom as an increase of the order parameter values was evidenced.