Abstract— The interaction of ionizing radiation and UV-light regarding colony forming ability in 3 diploid yeast strains of Saccharomyces cerevisiae was investigated. A wild type and two radiation sensitive mutants were used. No difference in the response of the three strains could be detected when the UV dose was given first, but when ionizing radiation was applied shortly before UV, there were essential differences depending on the kind of mutation. The involvement of repair mechanisms in the interaction is discussed.