Saccharin and contaminants of commercial Remsen-Fahlberg saccharin were studied for mutagenic potential with the use of the Salmonella/microsome test, Basc-test in Drosophila melanogaster and micronucleus test in mice. In none of these tests were mutagenic effects of saccharin observed. Likewise, the ortho- and para-sulfamoylbenzoic acids (OSBA and PSBA) were ineffective. Para-toluenesulfonamide (PTS) and the major contaminant ortho-toluene-sulfonamide (OTS) exhibited weak mutagenic effects in a modified Salmonella/microsome test and in Drosophila. These results do not indicate mutagenic and therewith correlated carcinogenic potential of saccharin, but they emphasize the possible activity of contaminants.