The genetic effects of methyl methanesulphonate (MMS) and bifunctional quinacrine mustard (QM) have been studied in three diploid strains of the yeast Saccharomyces cerevisiae: T1, with normal radiosensitivity, T2 - the excision-deficient mutant (rad2 rad2) and T3 - the mutant defective in recombinational repair (rad54 rad54). The strain T3 was much more sensitive to the lethal action of MMS than T1, but T2 did not differ from T1. The strain T2 was more sensitive to QM than T1 and T3. Both mutagens induce mitotic crossing over in T2 at a higher frequency than T1. MMS is not able to induce mitotic crossing over in T3 and QM demonstrates a very low induction. Treatment of the strains T1 and T2 with MMS and T1 with QM induces mitotic crossing over during the first cell division more often than during the second one. In most cases, QM induces mitotic crossing over in cells of the strain T2 during the second division. We suppose that the damages of DNA induced by QM in the wild type cells can be excised, but in the rad2 cells the gaps in DNA appeared after replication. In both cases, single-strand breaks of DNA are the main reason for mitotic crossing over.