P-Butoxyphenylacethydroxamic acid, benzohydroxamic acid, salicylhydroxamic acid, 2-naphthohydroxamic acid, indole-2-carbohydroxamic acid and benzoylaminoacethydroxamic acid were synthesized, and their mutagenicity for Salmonella typhimurium strains TA98 and TA100 were determined. Except for p-butoxyphenylacethydroxamic acid, all the hydroxamic acids were mutagenic for both strains. The mutagenicity progressed in the following order: 2-naphthohydroxamic acid greater than benzohydroxamic acid and salicylhydroxamic acid greater than benzoylaminoacethydroxamic acid and indole-2-carbohydroxamic acid. The starting materials for the synthesis of these acids including hydroxylamine were not in themselves mutagenic for TA98 and TA100. Thus, while the mutagenicity may require the hydroxamic acid as a whole, the acyl group may determine the mutagenic potency.