The major protein in isolated synaptic vesicles from bovine cerebral cortex has been compared to tubulin by sodium dodecyl sulphate-urea polyacrylamide gel electrophoresis, by two-dimensional gel electrophoresis, and by peptide mapping following limited proteolysis of the protein by Staphylococcus aureus protease. The results establish in purified synaptic vesicles the presence of tubulin, which is composed of the alpha and beta subunits. In the presence of ethyleneglycolbis)aminoethyl ether)-N,N'-tetraacetic acid (EGTA) or magnesium in the isolation buffers, the synaptic vesicles contained mainly the alpha-tubulin whereas the beta subunit was less abundant. Similarly, synaptosomal plasma membranes that were prepared in the presence of EGTA also contained more of alpha-tubulin than of the beta subunit. Non-ionic detergents such as Triton X-100 or Nonidet P-40 failed to solubilize the tubulin from the synaptic vesicles. Ionic detergents such as deoxycholate and sodium dodecyl sulphate solubilized all the vesicle proteins, including tubulin. The results indicate that alpha-tubulin is an integral vesicle membrane protein, whereas most of the beta subunit is peripherally attached and can be easily dissociated from the vesicle membrane with EGTA.