The microrheological characteristics of human blood cell suspensions flowing through very narrow glass capillaries (I.D. 5-12 microns) have been investigated using a traveling capillary method. In addition, red cell shape during flow was studied after cell fixation during capillary passage. The observations and measurements indicate the following: 1. The deformed shape of human red cells is not axisymmetrical and develops from an edge-on-oriented biconcave disc shape. 2. Red cells tend to travel in groups rather than equally spaced, presumably due to interindividual differences in size, shape, and deformability. 3. Red cell rotation in 8-12 microns capillaries occurs only if the flow forces are too small to induce cell deformation; during rotation of biconcave discs a substantial fraction of orbiting times is spent in an edge-on orientation; rotation is more frequently observed if the deformability of the erythrocytes is reduced. 4. Hematocrit-dependent transition from single-file to multi-file ('zipper') flow is observed in tubes whose inner diameter exceeds approximately 6 microns. 5. The flow pattern in the larger capillaries (8-12 microns) is characterized by significant variation of both radial and axial velocity of individual cells, as well as by concomitant changes of cell shape. However, cell-to-cell interactions and changes of cell orientation become progressively less significant in even smaller capillary tubes. 6. The bolus motion of intercellular plasma can be demonstrated by using microspheres or single platelets flowing between red cells as indicators. 7. Single platelets flowing between erythrocytes exhibit considerable variation in axial velocity in addition to rapid rotation.