An investigation was undertaken to determine whether analgesics and their derivatives (13 compounds), and anti-inflammatory drugs (4 compounds) had mutagenicity. Rec-assay was used to clarify specific DNA-damaging properties, and the Ames test was used to find back-mutations, using S-9 fractions obtained from the liver of 4 animal species pretreated with polychlorobiphenyl.In the Rec-assay, salicylic acid (2 mg), aspirin (5 mg), benzoic acid (4 mg), sulpyrine (0.4 mg), indomethacin (0.1 mg), oxyphenbutazone (0.1 mg) and diclofenac sodium (0.1 mg) showed a DNA-damaging tendency.In the Ames test, mutagenicity of methyl salicylate was demonstrated using the Salmonella typhimurium TA98 strain upon addition of hamster S-9 mixture. Weak mutagenicity was also found using the TA100 strain with rat S-9 mixture for salicylic acid, sulpyrine, indomethacin and oxyphenbutazone, and with hamster S-9 mixture for methyl salicylate, acetaminophen and phenacetine.