Abstract Nitrofurazone, nitrofurantoin, furazolidone, furaltadone and N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide (FANFT) were toxic to cultured mouse L cells. The extent of toxicity and the rate of reduction of nitrofurazone increased markedly as the oxygen content of the incubation medium was lowered. The toxic effect of nitrofurans was decreased by addition of serum and was much greater in phosphate-buffered saline containing glucose (PSG) than in medium. Damage to L cell DNA by nitrofurans increased as the oxygen concentration decreased from 21% to 0%. The concentration of nitrofurazone and duration of exposure also determined the number of DNA single-strand breaks. It is suggested that toxicity and DNA damage may result from the actions of toxic intermediates in the metabolic reduction of nitrofurans.