Erythrocyte flow dynamics in microvessels was quantitatively observed with an isolated rabbit mesentery. The flow resistance in the microvascular bed was analyzed with respect to the thickness of a cell-free layer along the inner vessel wall and the suspension viscosity of erythrocytes (in terms of hematocrit and erythrocyte deformability). The thickness of cell-free layer increased with the increase in the inner diameter of microvessels, by lowering the hematocrit, and by increasing the erythrocyte velocity, but it was constant in capillary region. The thickness decreased by decreasing the erythrocyte deformability with diamide, while it increased by accelerating the erythrocyte aggregation with dextran. The flow resistance in the microvascular bed was not simply related to the thickness of a cell-free layer, but it was well correlated to the viscosity of erythrocyte suspension. In the flow pattern of erythrocytes, the maximum inner diameter of microvessel required to induce parachute-like deformation decreased by decreasing the erythrocyte deformability. Moreover, the flow of erythrocytes became inhomogeneous and intermittent by accelerating the erythrocyte aggregation.