AbstractAddition of increasing amounts of zinc to a cold microtubule protein solution results in the disappearance of 30 S oligomer found in the absence of that cation and in the appearance of new tubulin oligomers, 90 S and 23 S. When a microtubule protein solution is warmed in the presence of zinc, tubulin‐sheets are assembled. We have tested the influence of microtubule associated proteins and the zinc:tubulin ratio on the polymerization process. Depletion of microtubule associated proteins results in wider and longer tubulin‐sheets than those polymerized in the presence of microtubule associated proteins. However by increasing zinc concentration wider but shorter tubulin‐sheets were found. These results suggest that microtubule associated proteins and zinc could promote nucleation of tubulin‐sheets, but zinc also promotes lateral tubulin‐tubulin interaction. This interpretation was confirmed when microtubule protein was assembled at a low zinctubulin ratio. In such conditions composite structures of microtubules and zinc tubulin‐sheets arc formed. These composite structures are consequence of a lateral attachment of a zinc tubulin‐sheet on a microtubule protofilament.