A series of aliphatic epoxides were tested for their ability to induce forward mutations in the yeast Schizosaccharomyces pombe. For all the compounds under study, a linear dose-response relationship was found, and the ranking of the relative specific activity was: epichlorohydrin greater than ethylene oxide greater than glycidol greater than 1,2-epoxybutane greater than 1,1,1-trichloropropylene oxide greater than propylene oxide greater than 2,3-epoxybutane. The influence of the metabolic conversion of the epoxides by the mouse-liver S9 fraction was also investigated. In such conditions, except for the 2,3-epoxybutane, the genetic activity of epoxides seems to be reduced.