This study evaluated the mutagenic potential of methyl disiloxane in the Ames assay using Salmonella typhimurium. Mutagenicity was evaluated by measuring the ability of methyl disiloxane to produce reverse mutations in several strains of Salmonella typhimurium (TA1535, TA1537, TA 1538, TA98, TA100) in the presence and absence of Aroclor-induced rat liver microsomal enzymes. The study consisted of two phases. The first phase was the spot plate test in which methyl disiloxane diluted in ethanol was spotted on individual plates of each strain, in the absence or presence of microsomal enzymes. In the second phase, concentrations of 0 (solvent control), 312.5, 625, 1250, 2500, or 5000 ug/plate, along with positive controls, were plated with each tester strain in the presence or absence of microsomal enzymes. Sodium azide, 9-amino acridine, 2-nitrofluorene, and 2-anthramine served as positive controls. No mutagenic responses were produced by the methyl disiloxane in either phase in any strain tested in the absence or presence of microsomal enzymes. Under the conditions of this study, methyl disiloxane was not mutagenic.