In an E. coli strain carrying two mutations, one in the dnaC gene involved in initiation of DNA replication and another in the uvrB gene which affects the excision-repair system, it has been shown that the SOS response cannot be induced by UV. This is probably due to the absence of any inducing signal (Salles and Defais, 1984). The capacity to induce the SOS network was followed using RecA protein amplification as a probe. When breaks were produced in DNA, RecA protein induction was restored. We describe here a strain in which both RecA protein and beta-galactosidase from a sfiA::lacZ fusion can be measured simultaneously in the same bacterial extract. In conditions in which no replication proceeds, this strain can be used to detect the ability of chemicals to produce free radical-mediated DNA breaks in vivo.