Platelet-rich rabbit plasma was incubated with surfaces of two different types--sheets of cultured mouse kidney epithelium and films of different lipids. The upper surface of epithelial sheets was found to be nonadhesive for platelets; in the same cultures, the platelets attached easily to the glass surface not covered by epithelial cells. Platelets did not attach to the surface of lipids (egg lecthin and dioleoyllecithin) that were in the liquid-crystalline state at 23 degrees C. In contrast, the surface of the films made of lipids (dipalmitoyl- and distearoyllecithin) that were in the crystalline state at 23 degrees C was adhesive for platelets. It had been found previously that the surfaces of epithelial sheets and of liquid lipid films are nonadhesive for fibroblasts. Possible mechanisms responsible for the nonadhesiveness of these surfaces are discussed. It is stressed that the factors responsible for nonadhesiveness of epithelial surfaces may be similar to those responsible for nonadhesiveness of the luminal surface of endothelium in blood vessels.