In this in vitro assessment of the mutagenic potential of Dow Corning 344 Fluid, histidine dependent auxotrophic mutants of Salmonella typhimurium (strains TAl535, TAl537, TA98 and TA100) and tryptophan dependent mutants of Escherichia coli (strains WP2 and WP2 uvrA) were exposed to the test substance diluted in ethanol, which was also used as a negative control. A preliminary toxicity test and two independent mutation tests were performed in the presence and absence of liver preparations from Aroclor 1254-induced rats (S9 mix). All the tests included a pre-incubation stage. Dose levels of up to 5000 ug/plate were tested in the preliminary toxicity test. This is the standard limit dose recommended in the regulatory guidelines this assay follows. No signs of toxicity were observed towards the tester strains so 5000 ug/plate was again chosen as the highest concentration in the main mutation test. Other dose levels used were a series of ca half-loglo dilutions of the highest concentration (1500, 500, 150 and 50 ug/plate). No evidence of mutagenic activity was seen at any dose level of Dow Corning 344 Fluid in either mutation test. The concurrent positive controls demonstrated the sensitivity of the assay and the metabolising activity of the liver preparations. It is concluded that, when tested in ethanol, Dow Corning 344 Fluid shows no evidence of mutagenic activity in this Bacterial Reverse Mutation Assay.