Human blood cells build a percolating physical gel all over the sample when at rest. This gel is progressively broken when it is continuously sheared in the bulk. It can slip at the wall, a phenomenon that depends on the roughness of the surface. Hence, smooth and rough walls were used td investigate the rheometrical shear properties of blood. A Couette-type rheometer with cylindrical walls allowed the shear rate to be varied in the range 10(-3)-10 s(-1) and the hematocrit in the range 0.53-0.95. Calibration was performed with standard silicon oil. The stress measured at low shear rates with rough walls seemed, indeed, to tend to a constant yield stress value. The value of the shear stress at the shear rate of 10(-3) s(-1) was taken as a realistic approximation of the yield stress of blood. This yield stress was measured for different values of cell concentration. Data were fitted and showed to be proportional to the cube of the concentration over the range of hematocrit studied. However, variability between the donors was observed. This may have relevance in the case of complete cessation of flow in vivo. The higher the yield stress of the static blood, the higher the driving pressure needed to get the flow started again.