Platelet membranes contain colchicine-binding activity which is not due to cytoplasmic contamination. This activity could be solubilized, at least partially, by Triton X-100 and to a lesser degree by Lubrol PX. Nonidet P-40 solubilized more colchicine-binding activity than was apparent in the intact, non-detergent-treated membranes, indicating that some of the protein was available for binding in the latter. Normal membranes had a far higher binding capacity for the alkaloid vinblastine than for colchicine, both of which were measured under equilibrium conditions. Membrane tubulin was also identified by acrylamide gel electrophoresis and immunologic precipitation. The protein had characteristics similar to the cytoplasmic form but was found to be more heat resistant. These findings establish with a high degree of certainty the presence of tubulin in the platelet membrane.