The mutagenic action of six compounds used in ELISA, EMIT and EIA assays, was investigated by means of the fluctuation test, with Klebsiella pneumoniae as a test organism, and the Ames' plate incorporation test using Salmonella typhimurium TA 98, TA 100 and TA 1537. It appears that 2,2'-azino-di(3-ethyl-benzthiazoline sulphonic acid (6)) or ABTS exerts mutagenic action on Klebsiella pneumoniae at a concentration of 11 g/l, on Salmonella typhimurium TA 100 at a top agar concentration of 0.1 g/l, on Salmonella typhimurium TA 98 at 0.2 g/l and on Salmonella typhimurium TA 1537 at 10 g/l. With umbelliferone, mutagenic action was found only with Klebsiella pneumoniae at a concentration of 0.8 g/l or higher. With o-phenylenediamine, strong mutagenic activity was found only with strain TA 98 and metabolic activation at a top agar concentration of 0.001 g/l. With 5-aminosalicylic acid, beta-methylumbelliferone and p-nitrophenyl-phosphate, no mutagenic action was observed.