The dependence of the yield of mutations of the resistance to 8-azaguanine induced with 0-methylhydroxylamine on the number of cell generations that have passed by the time of the creation of selective conditions and the existence of a phenotypic lag period is established. The highest rate of mutations is found after 8-azaguanine treatment in two generations. As the number of generations increased, the number of detectable mutations decreased. A dependence is observed between the induced mutation frequency and death of cells as the latter are transferred to selective conditions after two generations. In experiments on induced mutagenesis, one can obtain more exact quantitative characteristics by introducing a selective agent into the cell suspension but not by treating growing cells in colonies.