In budding yeast, the essential roles of microtubules include segregating chromosomes and positioning the nucleus during mitosis. Defects in these functions can lead to ancuploidy and cell death. To ensure proper mitotic spindle and cytoplasmic microtubule formation, the cell must maintain appropriate stoichiometries of [alpha]- and [beta]-tubulin, the basic subunits of microtubules. The experiments described here investigate the minimal levels of tubulin heterodimers needed for mitotic function. We have found a triple-mutant strain, pac10[DELTA] plp1[DELTA] yap4[DELTA], which has only 20% of wild-type tubulin heterodimer levels due to synthesis and folding defects. The anaphase spindles in these cells are ~64% the length of wild-type spindles. The mutant cells are viable and accurately segregate chromosomes in mitosis, but they do have specific defects in mitosis such as abnormal nuclear positioning. The results establish that cells with 20% of wild-type levels of tubulin heterodimers can perform essential cellular functions with a short spindle, but require higher tubulin heterodimer concentrations to attain normal spindle length and prevent mitotic defects.