OBJECTIVE The deformation of erythrocytes in microvessels less than 15 microns in inner diameter was analyzed using a microvascular bed isolated from rabbit mesentery. The deformation was compared with that found in glass capillaries. METHODS Human erythrocytes were perfused through two media: first, a microvascular-bed section isolated from rabbit mesentery; and second, a set of glass capillaries. Images of deformed erythrocytes were recorded on videotape under strobe light and analyzed with an image processor. The flow velocity of the erythrocytes was determined from the difference of their positions between video frames or by a dual-spot cross-correlation technique. Erythrocyte deformability was modified with diamide, diazene dicarboxylic acid bis[N,N-dimethylamide], by crosslinking spectrins. RESULTS Symmetrical (parachute-like or slipper-like) deformation of erythrocytes was observed only in microvessels smaller than 13 microns in inner diameter. Erythrocytes in microvessels were less deformed than those in glass capillaries with corresponding diameters, and the marginal cell-free layer was narrower. The deformation increased by increasing the flow velocity of erythrocytes, and the cell-free layer became wider. Diamide-treated cells in microvessels were less deformed than normal cells and showed slightly narrower cell-free layers. Stronger stress in narrower microvessels induced further deformation of cells. CONCLUSIONS Erythrocyte deformation in microvessels was essentially different from that in glass capillaries, and the effect of erythrocyte deformability on the flow dynamics of erythrocytes in microvessels was properly evaluated using an isolated microvascular bed.