This study was undertaken to evaluate the mutagenicity of the flame retardant, Tetrakis (Hydroxymethyl) Phosphonium Sulfate -75% or its possible metabolic products using a battery of in vitro and in vivo assays. The starting material in the fabric treatment process, THPS was tested with five strains of Salmonella typhimurium with and without the addition of a rat liver activating system. In addition, the THPS and the treated fabric were assayed in a combined testing protocol. The mutagenicity tests which were employed in this protocol included (a) the analysis of the urine of treated mice, (b) the micronucleus test, and (c) metaphase analysis. The animals were administered the THPS both orally and dermally and the treated fabric was incorporated into the animals' feed. The THPS produced no mutagenic response in any of the strains of S. typhimurium either when tested directly or with the addition of Acroclor--or phenobarbital-- induced rat liver. The results of the combined testing protocol, with the three routes of administration, were also negative for the urine analysis, micronucleus test and metaphase analysis.