The toxicity of 2,4,6-trinitrotoluene (TNT), a widespread environmental contaminant, is exerted through its enzymatic redox cycling and/or covalent binding of its reduction products to proteins and DNA. In this study, we examined the possibility of another cytotoxicity mechanism of the amino- and hydroxylamino metabolites of TNT, their flavoenzyme-catalyzed redox cycling. The above compounds acted as redox-cycling substrates for single-electron transferring NADPH:cytochrome P-450 reductase (P-450R) and ferredoxin:NADP(+) reductase (FNR), as well as substrates for the two-electron transferring flavoenzymes rat liver NAD(P)H:quinone oxidoreductase (NQO1) and Enterobacter cloacae NAD(P)H:nitroreductase (NR). Their reactivity in P-450R-, FNR-, and NR-catalyzed reactions increased with an increase in their single-electron reduction potential (E(1)(7)) or the decrease in the enthalpy of free radical formation. The cytotoxicity of the amino- and hydroxylamino metabolites of TNT towards bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) was partly prevented by the antioxidant N,N'-diphenyl-p-phenylene diamine and desferrioxamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea, thus pointing to the involvement of oxidative stress. In general, their cytotoxicity increased with an increase in their electron accepting properties, or their reactivity towards the single-electron transferring FNR and P-450R. Thus, our data imply that the flavoenzyme-catalyzed redox cycling of amino and hydroxylamino metabolites of TNT may be an important factor in their cytotoxicity.