Methyl 3,4-diphenyl-5-hydroxylamino-2-furoate (N-OH-MDPF) (I), methyl 3,4-diphenyl-5-acetoxyamino-2-furoate (N-OAc-MDPF) (II), methyl 3,4-diphenyl-N-hydroxy-5-acetylamino-2-furoate (N-OH-MDPAF) (III), and methyl 3,4-diphenyl-N-acetoxy-5-acetylamino-2-furoate (N-OAc-MDPAF) (IV) were synthesized and tested for mutagenic activity for Salmonella typhimurium TA98 and TA100. The hydroxylamine (I) and acetyl derivatives (II-IV) did not show mutagenic activity in TA98 or TA100. In contrast, the parent nitro compound, methyl 3,4-diphenyl-5-nitro-2-furoate (MDPNF) (V) was found to be equally active in TA98 and TA98-DNP, and more active in TA100 and TA104. The mutagenic activity in TA100 and TA104 decreased significantly under anaerobic conditions. Additionally, MDPNF was previously shown to be less mutagenic in the nitroreductase-deficient derivatives TA100NR and TA98NR, suggesting a requirement for nitro reduction. Incubation of V with NADPH and bacterial lysates of TA98 or TA98NR yielded a metabolite which was identified as I based on chromatographic and mass spectral characteristics. The rate of reduction by the lysate of TA98NR was about one-third that of TA98, showing a correlation between mutagenicity and nitroreductase activity. The lysates of TA98 did not reduce N-OH-MDPF further to the amine. In contrast to the lack of mutagenic activity of I-IV, N-hydroxy-4-aminobiphenyl (N-OH-ABP) and its acetyl derivatives were active in TA98, but less so in TA98-DNP. These data suggest that mechanisms involving O-acetylation of N-hydroxylamine to the acetoxyamine or acyl transfer reactions are not involved in the generation of mutagen from MDPNF. Furthermore, the differential mutagenic response of V in TA98 and TA98NR, its reduction to I, and the lack of activity of I suggest that the intermediates of reduction between the nitro and hydroxylamine, such as nitro or nitroso free radical anions, may be involved in mutagenesis. The decreased response of V under anaerobic conditions and increased response in TA104 suggest that secondary oxygen radicals generated from reduction intermediates may be responsible for the mutagenicity of MDPNF.