The SOS-function-inducing activities of 36 furylethylenes were characterized in Escherichia coli K12. The induction of the SOS function was assayed by monitoring the beta-galactosidase activity in the sulA::lacZ fusion strain PQ 37. To correct for the inhibitory effects of test compounds on mRNA or protein synthesis, the level of the constitutive alkaline phosphatase was assayed in parallel. Tested furylethylenes included nine alkylesters and eleven N-alkylamides of 5-nitro-2-furylacrylic acid (NFAA) and fourteen derivatives differing not only in substituents at exocyclic double bond, but also in the position 5 of the furan ring. The induction of the SOS-function by the derivatives depends on the presence of 5-nitrofuran centre in their molecule; side chains in the position 2 modify the degree of SOS response. SOS-inducing potency of n-alkyl congeners decreases with increasing lipophilicity. Effect of derivatives with branched alkyl substituents is lower than expected from the behavior of the n-alkyl homologues. All derivatives with positive effect on SOS-function in E. coli show mutagenic activity on Salmonella typhimurium TA98 in Ames test.