Three new genes affecting microtubule function in Saccharomyces cerevisiae were isolated by screening for mutants displaying supersensitivity to the antimicrotubule drug benomyl. Such mutants fall into six complementation groups: TUB1, TUB2 and TUB3, the three tubulin genes of yeast, and three new genes, which we have named CIN1, CIN2 and CIN4. Mutations in each of the CIN genes were also independently isolated by screening for mutants with increased rates of chromosome loss. Strains bearing mutations in the CIN genes are approximately tenfold more sensitive than wild type to both benomyl and to the related antimicrotubule drug, nocodazole. This phenotype is recessive for all alleles isolated. The CIN1, CIN2 and CIN4 genes were cloned by complementation of the benomyl-supersensitive phenotype. Null mutants of each of the genes are viable, and have phenotypes similar to those of the point mutants. Genetic evidence for the involvement of the CIN gene products in microtubule function comes from the observation that some tubulin mutations are suppressed by cin mutations, while other tubulin mutations are lethal in combination with cin mutations. Additional genetic experiments with cin mutants suggest that the three genes act together in the same pathway or structure to affect microtubule function.