The mutagenicity of ethyl 1,2,3-triazino[5,4-b]indole-4-carboxylate N(3)-oxide (D3) and 2-chloroethyl 1,2,3-triazino-[5,4-b]indole-4-carboxylate N(3)-oxide (D4), heads of series of new products with considerable platelet antiaggregating and hypotensive activity, and their precursors 2-ethoxy-carbonylmethyl-1-methylindole-3-carboxylic acid (A3) and 2-(2-chloroethoxycarbonylmethyl)-1-methylindole-3-carboxylic acid (A4) were tested in four strains of Salmonella typhimurium (TA98, TA100, TA97 and TA102) using the standard plate incorporation technique. A3 and A4 were not mutagenic whereas D3 was mutagenic to all the strains and D4 was mutagenic to TA97, TA98 and TA100. The addition of 4 or 10% of S9 mix decreased the mutagenic activity of both compounds. This effect was independent of the concentration of S9 in the S9 mix.