Platelet diffusivity was measured in flowing blood by a technique based on the classical experiment of Taylor. Values of diffusivity were calculated from experimental data with the aid of theoretical results obtained by Ananthakrishnan, et al. Values ranged between 0.5 and 2.5 × 10−7 cm2/sec for a series of hematocrits (H = 0-50) and shear rates ([formula omitted]). These values imply significant enhancement of diffusivity due to red cell motion. The experiment was not sensitive enough to detect the dependence of diffusivity on either shear rate or hematocrit, although on the assumption of power-law dependence of diffusivity on shear rate the data indicated a power-law coefficient of less than 0.5. At high shear rates (>200 sec!1) results were found to be influenced by nonrandom cell migration; however, upon correction for this migration, these results were consistent with the results obtained at low shear rate.