The conditions for assembly-disassembly of platelet microtubules were evaluated using a highly purified tubulin preparation obtained from human platelets. The extent of polymerization could be measured by recording the increase in optical absorbance at 500 nm. Electron microscopy verified the tubular nature of the polymerized tubulin. Maximal polymerization was directly proportional to the protein concentration and the temperature of the system. Rate and magnitude of polymerization at 37°C was significantly increased by addition of GTP or ATP. Cyclic nucleotides did not influence upon this process. Storage at 4°C rapidly decreased the ability of tubulin to polymerize. The method was developed to evaluate a dynamic equilibrium in vivo between microtubules and subunit protein (tubulin) of which they are formed. Chilled platelets showed significantly decreased amount of microtubules concomitantly associated with increased tubulin. It is suggested that this method could be widely utilized to evaluate such a shift of equilibrium in platelets, which may affect several important platelet functions.