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To determine the shear force acting on a white blood cell sticking to the endothelium of a blood vessel, the flow field about a single white blood cell in a venule was determined by hign-speed motion picture photomicrography. The force acting on the white blood cell was then calculated according to the principles of fluid mechanics. In this paper, the calculation was made using an experimentally determined dimensionless shear force coefficient obtained from a kinematically and dynamically similar model. The large physical model of the hemodynamic system could be easily instrumented, and the shear force acting on the model cell and the flow field around it were measured. The data were then used to calculate a shear force coefficient. On the basis of dynamic similarity, this shear force coefficient was applied to the white blood cell in the venule. The shear force coefficient was strongly influenced by the hematocrit, so in vivo hematocrits were measured from electron micrographs. It was found that in the venules of the rabbit omentum a white blood cell sticking to the endothelial wall was subjected to a shear force in the range of 4 times 10--5 dynes to 234 times 10--5 dynes; the exact value depended on the size and motion of the white blood cell, the size of the blood vessel, the velocity of the blood flow, and the local hematocrit, which varied between 20% and 40% in venules of about 40 mum in diameter. The contact area between the white blood cell and the endothelial cell was estimated, and the shear stress was found to range between 50 dynes/cm-2 and 1060 dynes/cm-2. The normal stress of interaction between the white blood cell and the endothelium had a maximum value that was of the same order of magnitude as the shear stress. The accumulated relative error of the experimental procedure was about 49%. The instantaneous shear force was a random function of time because of random fluctuations of the hematocrit.