The range of specificity of the rev2-1 mutation, an allele that reduces the frequency of ochre revertants induced by UV in Saccharomyces cerevisiae (LEMONTT 1971a), has been investigated by examining its influence on the reversion of eleven well-defined and contrasting cyc1 mutations. We have shown, in support of a suggestion of LEMONTT (1971a), that the REV2 gene product is concerned only with the reversion of ochre alleles; it plays virtually no role in the reversion of amber, missense or frameshift mutations. We have also shown that its effect is specific and confined to only some highly revertible ochre alleles. The REV2 gene product appears to enhance reversion at these sites by facilitating the conversion of two otherwise nonmutagenic photo-products into a single premutational lesion. UV-induced killing of rev2-1 strains was found to be significantly greater on fermentable rather than on nonfermentable media.