The yeast Saccharomyces cerevisiae provides a model eukaryotic system with which to assess the genetic effects of physical and chemical carcinogenic agents during mitosis and meiosis. Our previous studies of the mechanisms of spontaneous mitotic and meiotic recombination have led to the development of genetic techniques which we employ in the present experimental program to characterize the effects of X-rays, ultraviolet light, ethyl methanesulfonate and ethylnitrosourea on gene mutation, intragenic recombination, intergenic recombination, and chromosomal nondisjunction and/or loss following exposure of diploid hybrids to these agents during mitosis and meiosis. Synchronous mitotic and meiotic populations will be employed to determine whether these agents exert their genetic effects at discrete times during mitosis and meiosis. The mitotic studies proposed employ MATalphaMATalpha hybrids and congenic MATalphaMATalpha and MATalphaMATalpha strains. The use of diploids homozygous at MAT facilitates the genetic analysis of putative nondisjunctants. The analysis of neiotic populations focuses upon full genetic characterization of the various cell types present following exposure of sporulating cultures to mutagen-recombinogens, including mitotic diploid cells, meiototic diploid cells and ascospores. We anticipate that a comparative study of this type will contribute to further understanding of the mechanisms of mitotic versus meiotic induced mutation, recombination and chromosomal nondisjunction.