Triploid and tetraploid strains of Saccharomyces cerevisiae were constructed and the spontaneous loss during mitosis of one, two or three copies of chromosome VII was determined. In one strain, a triploid (VM2) in which expression of the recessive alleles can be observed only after loss of two copies of chromosome VII (3N-2), the spontaneous frequency of chromosome loss was lower than in the diploid D61.M. In another strain, a tetraploid (VM4) that also requires the loss of two copies of chromosome VII for observation (4N-2) of the recessive alleles, the spontaneous frequency was slightly higher than in the diploid D61.M. The spontaneous frequency of other genetic events (that is, mutation, recombination or chromosome breakage) were lower by 2-3 orders of magnitude than in the diploid strain D61.M. Induction of chromosome loss and other genetic events by nocodazole, ethyl acetate, hydroxyurea and ethyl methanesulfonate was determined in D61.M, VM2, and VM4, and the results were compared. Nocodazole and ethyl acetate induced chromosome loss in both the triploid and the tetraploid strains at lower concentrations than required in the diploid. These compounds also induced elevated frequencies of other genetic events in both the triploid and the tetraploid strains but not in the diploid. Hydroxyurea induced elevated frequencies of chromosome loss in the diploid and the tetraploid. Frequencies of chromosome loss in the triploid treated with hydroxyurea, although elevated, are based on observation of very few colonies of the correct phenotype. Ethyl methanesulfonate failed to induce chromosome loss in any of the three strains. Hydroxyurea and ethyl methanesulfonate did, however, induce very high frequencies of other genetic events.