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 "mutation-mitotic segregation" mechanism, while HAP induces mutants through novel mechanism "both allele mutation". Manifestation of such mechanism is the null fertility after meiosis of diploid mutants induced by HAP.