The purpose of this study was to evaluate the genetic activity of dimethyl siloxane fluid. The fluid, diluted with dimethyl sulfoxide, was evaluated in Salmonella typhimurium and Escherichia coli reverse mutation assays as outlined in O.E.C.D. (CRF 46 (162)). Salmonella typhimurium (TA1535, TA97, TA98, and TA100) and Escherichia coli (WP2) were plated with dimethyl siloxane fluid concentrations of 312.5, 625, 1250, 2500, or 5000 ug/plate, or with positive or negative controls, both with and without microsomal activation by Aroclor-induced rat liver enzymes and were incubated for 72 hours. Dimethyl sulfoxide served as the negative control, while sodium azide, 4-nitroquinoline-n-oxide, daunomycin. n-methyl- n-nitro-n-nitrosoquanidine, 2-anthramine, and 2-aminofluorene served as positive controls. No evidence of mutagenicity was observed for the test material.