Particle deposition onto filtering surfaces is expected to occur when the drag force of filtration overcomes the fluid mechanic wall repulsive force. A nondimensional particle deposition parameter has been formulated and experimentally validated utilizing red blood cells. The parameter is applied to platelet deposition in the vasculature and shows that platelets are expected to contact arterial surfaces only in low shear, separated regions. This may be the link between the filtration and thrombosis theories of atherosclerosis. Platelet radius and plasma viscosity are predicted to be important parameters of atherogenesis.