Homogeneous mutagenicity data, available for 16 hydrazine derivatives assayed in the Salmonella/microsome test, were tentatively associated with their chemical structure. Some possible relationships were detected between chemical features and the following parameters in vitro: (a) mutagenic potency, expressed as revertants per micromole compound, which varied over a 3200-fold range without S-9 mix and a 7000-fold range with S-9 mix; (b) reversion mechanism, as inferred from the selective sensitivity of 5 Salmonella typhimurium tester strains; (c) metabolic behaviour in the presence of S-9 mix containing rat liver, mouse liver or mouse lung /-9 fractions from animals treated with Aroclor-1254.