Abstract Platelet transport theory is based on convection diffusion and describes adequately the influence of wall shear rate, platelet concentration and axial (down stream) position. Until now, the influence of the predominant factors affecting platelet adherence, the hematocrit and the red cell size, was not included in this theory. Their role remained hidden in the platelet diffusivity ( D w ), which was assumed to be related to the shear rate (γ) expressed in s −1 by a power law function D w = mγ n , in which m and n were thought to be constants. We have determined platelet diffusivity directly from platelet adherence to subendothelium as a function of axial distance in an in vitro perfusion system. Our results indicate that m is a constant with a value of (1.05 ± 0.05) 10 −9 cm 2s −1 and that n is a function of the hematocrit ( h) which is best approximated by a quadratic equation n = 0.297 + 1·29 h − 0.90 h 2. The effect of red cell size was introduced by correcting the hematocrit containing factors in this quadratic equation for the square of the red cell diameter. This correction was made on the basis of theoretical considerations. The theoretically derived platelet adherence correlated closely with the previous experimental data regarding the effect of red cell size in which we found that the hemodynamic effect of red cells on platelet adherence decreases with decreasing red cell diameter.