A range of physical and chemical agents induce the mitochondrial 'petite' mutation in the yeast Saccharomyces cerevisiae. DNA intercalating agents as well as chemicals which can interfere with DNA synthesis induce this mutation, but only in growing cells. Many chemical or physical agents that produce a DNA lesion which is not simply reversed can induce various levels of the petite mutation, and may be more effective in non-growing cells. A limited number of chemicals act like ethidium bromide, inducing a high frequency of petites which is partially reversible with increasing concentration or time. The ability of a specific compound to be transported into mitochondria or its affinity for AT base pairs in DNA may determine whether it acts primarily as a nuclear or mitochondrial mutagen. In mammalian cells, some neoplastic changes occur at the mitochondrial level. Analogies between yeast and mammalian mitochondria suggest that agents which increase petite mutagenesis in yeast may have some carcinogenic potential. Although some types of petite inducer may have potential as antitumour drugs, those which are very effective antimitochondrial agents appear to be too toxic for therapeutic use. A process comparable to early stages in petite mutagensis occurs in human degenerative diseases and it seems possible that a consequence of exposure to petite mutagens could be an increase in the rate of degenerative diseases or of the aging process.