The inactivation of three radiation-sensitive haploidSaccharomyces mutants, as well as that of corresponding homozygous diploid mutants, following treatment with the bifunctional alkylating agent diepoxybutane (DEB) or the monofunctional alkylating agents methylmethane-sulfonate (MMS) or methylnitrosoguanidine (MNG) was investigated. Sensivity to DEB was correlated with sensitivity to ultraviolet light. The response to MMS or MNG treatment was more complex. The most UV-sensitive haploid and diploid mutants were as resistant as wild type after MMS or MNG treatment. Those mutants with an inter-mediate UV sensitivity were also sensitive after MMS treatment, independent of their state of ploidy. After exposure to MNG, however, a characteristic difference appeared between the sensitivity of the two intermediate sensitive haploids and the corresponding diploids. A model is discussed, interpreting the results obtained as an indication of the existence of at least three different repair mechanisms inSaccharomyces.