Microtubule-associated proteins (MAPs) promote the assembly of microtubules from purified tubulin in vitro. In order to establish a model system for the investigation of the role of MAPs in micro tubule assembly in vivo, we have generated Saccharomyces cerevisiae strains that permit the modulation of the expression levels of MHP1 (MAP-Homologous Protein 1) and of the α and β-tiibulin genes. Simultaneous overexpression of α and β tubulin results in the accumulation of long aberrant microtubules in interphase, a similar phenotype as was observed in cells overexpressing MHP1. We demonstrate that overexpression of MHP1 in asynchronously growing yeast cultures leads to cell cycle arrest in G2. In cells that overexpress MHP1 and the tubulin genes, a suppression of both the MHP1 and the tubulin overexpression phenotypes can be observed. Progressive induction of α and β tubulin overexpression and constitutive overexpression of MHP1 lead to the formation of long cytoplasmic microtubules more frequently than observed in cells overproducing tubulin or Mhplp individually and the increased microtubule polymerization could be correlated with the increase of α and β tubulin expression. However, the overexpression of MHP1 did not alter the phenotypes of individual overexpression of α or β-tubulin. These data indicate that Mhplp not only stabilizes microtubules but promotes microtubule assembly in vivo, and suggest that the role of other mammalian MAPs in the promotion of microtubule assembly could be tested in this yeast system.