Chemical mutagens 6-N-hydroxylaminopurine (HAP) and propiolactone (PRO) induce Lys2 mutants with high frequency in diploid yeast Saccharomyces cerevisiae. HAP induces such mutants even in tetraploid strains. The genetic analysis of mutants was performed. It is shown that PRO induces mutants by means of Xmutation-mitotic segregationX mechanism, while HAP induces mutants through novel mechanism Xboth allele mutationX. Manifestation of such mechanism is the null fertility after meiosis of diploid mutants induced by HAP.