Nitrososarcosine, not mutagenic itself in the host-mediated assay using Salmonella typhimurium G46 as indicator organism, lowered the mutant frequency produced by dimethylnitrosamine (DMN). Mutant frequency was significantly depressed when 1.0 g/kg nitrososarcosine was administered by gavage 0.5--2.0 h prior to intramuscular injection of 500 mg/kg DMN. Doses of nitrososarcosine as low as 37.2 mg/kg administered 45 min prior to dimethylnitrosamine treatment produced statistically significant reduction of mutant frequency. Dimethylnitrosamine, diethylnitrosamine and dibutylnitrosamine (500 mg/kg) also partially suppressed the mutant frequency produced by 500 mg/kg DMN when administered 45 min prior to dmn. diethylnitrosamine and dibutylnitrosamine were not found to be mutagenic in this test system.