The genotoxicity of a variety of hydrazine derivatives was examined in the DNA-repair test on rat or mouse hepatocytes. Out of 32 hydrazine derivatives, 6 chemicals, i.e., N'-acetyl-4-(hydroxymethyl)phenylhydrazine, 1,2-dimethylhydrazine.2HCl, 1-hydrazinophthalazine.HCl, methylhydrazine.sulfate, p,p'-oxybisbenzene disulfonylhydrazide and phenylhydrazine.HCl, elicited positive DNA repair responses in the test on rat hepatocytes. In the test on mouse hepatocytes, 4 more hydrazine derivatives, i.e., 1,1-dimethylhydrazine, hydrazine hydrate, hydrazine sulfate and 2-methyl-4-chlorophenoxyacetic acid hydrazide.HCl also generated positive responses, in addition to the 6 positive compounds in the rat assay. These results suggest that mouse hepatocytes are more susceptible to the genotoxicity of hydrazine derivatives, and that the species differences in genotoxicity appear to be in agreement with the in vivo carcinogenicity of these agents.