1,2,3-Tribromopropane, 1,2,3-trichloropropane, and 1,2-dibromo-3-chloropropane are mutagenic in strains TA1535 and TA100 of Salmonella typhimurium, but only in the presence of rat liver S9 mix. This requirement for metabolic activation was unexpected for an alkyl halide and thus suggested the metabolic formation of the 2-keto derivatives (di-haloacetone). The 2-methyl derivatives of the halopropane compounds did not induce a doubling of revertants compared to controls. It was demonstrated that none of these compounds is converted into a secondary material that could be determined as structurally different by gas chromatography. These observations suggest that lack of mutagenicity of the methylated compounds is a manifestation of a steric effect.