Hematocrit and red cell size are important factors for the transport of blood platelets toward subendothelium in flowing blood. We report that red cell deformability also influences platelet transport. Red cell deformability was estimated with Couette-flow viscosimetry at a shear rate of 130 s-1 and expressed as a ‘T’ factor--a dimensionless parameter relating the relative viscosity and the hematocrit derived from the relationship: T = (1 - mu -0.4 rel)/H, where mu rel is the relative viscosity and H is the hematocrit. The normal value of T was within a narrow range (0.91 +/- 0.02). Treatment of normal red cells with isoxsuprine and chlorpromazine caused decreased rigidity and decreased T. Cholesterol loading and treatment with diamide increased rigidity and increased T. In vitro perfusion experiments in an annular perfusion system with everted human umbilical arteries were performed with perfusates to which such treated red blood cells were added to investigate their influence on platelet adherence to artery subendothelium. Platelet adherence was well correlated with red cell rigidity, with increased adherence at increased rigidity and vice versa. A change in T of 0.10 corresponded to a change in platelet adherence of approximately 50%. These effects were more pronounced at a wall shear rate of 1,800 s-1 than at 300 s-1.