The mutagenicity of halothane was tested In an <in-vitromicrobial assay system employing two histidine-dependent mutants of Salmonella typhimurium, TA9S and TA100. Halothane in concentrations ranging from 0.1 to 30 per cent was incubated with bacteria in the presence or absence of a metabolic activation system prepared from either nit liver treated with Aroclur 1254 or human liver. Trifluoroacelic acid, a major metabolite of halothane, and urine from patients anesthetized with halothane also were tested. Halothane, trifluoroacetic acid, and patients* urines were not mutagenic.