Nifurpirinol, 6-hydroxymethyl-2-[2-(5-nitro-2-furyl)vinyl]pyridine, which has been widely used as an antibacterial drug against diseases of fish, was found to be a potent mutagen. The mutagenic effect was dose-related, and the potencies were calculated to be 1.49 X 10(3), 7.24 X 10(4) and 8.49 X 10(5) revertants/1 X 10(-8) survivors at the concentration of 1 microgram/ml to :Escherichia coli B/r WP2, E. coli B/r WP2 hcr- and Salmonella typhimurium TA100, respectively, without metabolic activation. Nifurpirinol is susceptible to photodegradation, and its mutagenic activity to S. typhimurium TA100 was decreased to about one thousandth when the solution of Nifurpirinol was exposed to sunlight at room temperature for 4 h.