Erythrocyte aggregation is a normal physiological phenomenon, which is especially confined to the microcirculatory networks. It is the major determinant of blood viscosity, particularly under conditions of low shear stress in the microcirculation so affecting the flow dynamics that may be the cause of many pathological conditions. This study was done to show the effect of different membrane constituents on the aggregation kinetics. Thirty two young healthy adults were included in this study (23 males and 9 females). 12 ml of fresh venous blood was taken from each subject. Physiological measurement included ESR (Westergren method), PCV; erythrocyte deformability (by simple filtration technique). Erythrocyte aggregation and sedimentation was measured by He-Ne laser scattering technique using dextran 500 in concentration of 2.5 g % as an aggregating macromolecule. Biochemical measurement included at first isolation of the membrane, then measurement of different membrane structural and functional constituents (sialic acid, protein, cholesterol, phospholipids, Na+-K+ ATPase activity, Ca++ ATPase activity and Mg++ ATPase activity). There was no significant correlation between ESR and aggregation and sedimentation parameters measured by He-Ne laser scattering technique except with R3DS. The effect of each membrane constituents on aggregation and sedimentation parameters was considered separately and the value of each of them were divided into;three groups (low, medium and high). The difference between medium and high membrane sialic acid concentrations significantly increased aggregation time (AT) and decreased rate of aggregation (RA), three-dimensional aggregation time (3DAT) and sedimentation time (ST). Protein showed a Diphasic' effect, there was significant increase in one-dimensional aggregation time (1DAT), 3DAT and ST with the difference between low and medium concentrations, and a highly significant decrease in 3DAT with the difference between medium and high protein concentrations. Cholesterol concentration difference between low and medium caused a highly significant decrease in 3DAT and ST and an increase in rate of three-dimensional aggregate sedimentation (R3DS). Phospholipid concentration difference between low and medium significantly decrease in 1DAT, while the difference between medium and high concentration significantly decrease AT, ID AT and 3DAT and increase R3DS. Cholesterol/phospholipids ratio difference between low and medium significantly decrease 3DAT and increase R3DS. Na+-K+ ATPase activity difference between low and medium significantly increase 3DAT, ST and RA. While the difference of activity between medium and high significantly decrease ST and increase RA. Ca++ ATPase activity difference between low and medium significantly increase 3DAT and decrease rate of one-dimensional aggregate sedimentation (RIDS) and R3DS. While the difference of activity between medium and high significantly decrease RA and RIDS. Variation of Mg++ ATPase activity within normal limits showed insignificant effect on aggregation and sedimentation parameters. Deformability values expressed in term of rigidity index, the difference between medium and high rigidity index significantly decrease the RA and R3DS. However, variation of concentration of different membrane constituents showed insignificant effect on deformability values.