Seventeen different reports are available dealing with the mutagenic effects of saccharin. Many of these are short abstracts, carrying incomplete information. Mainly tested as its sodium salt, saccharin has been found to be weakly mutagenic in Salmonella at very high doses, in Drosophila at moderate doses, and in mice at moderate to high doses. The compound is a weak chromosome breaker in onion root tips and in Chinese hamster cells. For most of these, and for other test systems as well, a number of doubtful or negative results have also been reported. Altogether the evidence for chromosome-breaking properties is stronger than for the induction of point mutations. The overall picture is too conflicting and equivocal to classify saccharin as a proven mutagen. It is suggested that the observed contradictions might be related to the occurrence of varying amounts of impurities.