The mutagenic action of 3 coccidiostatic chinoxaline-N-oxide derivatives, quindoxin, carbadox and olaquindox, was investigated by Luria and Delbrück's fluctuation test, with Klebsiella pneumoniae and Escherichia coli K12 as test organisms. These compounds were mutagenic at very low concentrations (2 X 10(-5)--500 X 10(-5) mmole/l). In the Ames test they showed a mutagenic action without metabolic activation with Salmonella typhimurium TA98 and TA100 at concentrations of 0.001-0.1 mmole/l in the top agar. Hence, these compounds cause both base-pair substitutions and frame-shift mutations. When Saccharomyces cerevisiae D4 was cultivated in the presence of the compounds, an increase in the mitotic gene conversions was observed. Certain other N-oxides also showed a mutagenic action in the fluctuation test. With Klebsiella pneumoniae, 4-nitroquinoline 1-oxide was mutagenic at a concentration of 0.005 mmole/l, quinoline 1-oxide at 10 mmole/l and benzofuroxan at 0.01 mmole/l. In this test no mutagenic action was found with 4-nitropyridine 1-oxide, pyridine 1-oxide or 4-picoline 1-oxide. With Salmonella typhimurium TA98 and TA100, 4-nitroquinoline 1-oxide, benzofuroxan and 4-nitropyridine 1-oxide were mutagenic, whereas quinoline 1-oxide, pyridine 1-oxide and 4-picoline 1-oxide were not. In contrast, with the fluctuation test, 4-nitroquinoline 1-oxide appeared to be more mutagenic than quindoxin, carbadox and olaquindox in the plate incorporation test.