2-Amino-3-methylimldazo [4,5- f ]quinoline [I] is a potent mutagen isolated from broiled sun-dried sardine. [I] and its seven of derivatives, (two isomers, one demethylated derivative and four methyl-substituted derivatives), were tested for mutagenicity on Salmonella typhimurium TA98 and TA100 in the presence of S9 mix. 2-Amino-1,4-dlmethylimldazo [4,5- f ]quinoline was the strongest mutagen of these 8 compounds on TA98, giving 159,000 revertants/nmol (750,000 revertants/μg). The demethylated derivative, 2-aminoimidazo [4,5- f ]quinoline, had very weak mutagenicity, inducing only 55 revertants/nmol (200 revertants/μg). Compounds having a methyl group at position N-1 or N-3 of 2-aminoimidazo [4,5- f ]quinoline were strong mutagens. The 1,5-dimethyl-derivative was more mutagenic than 3,5-dimethyl-derivative. Introduction of a methyl group at position 4 and position 5 enhanced and reduced the mutagenicity, respectively. All the compounds tested were more mutagenic to TA98 than to TA100, but their relative orders of mutgenlcity with TA98 and TA100 were the same.