Escherichia coli responds to impairment of DNA synthesis by inducing a system of DNA repair known as the SOS response. Specific genes are derepressed through proteolytic cleavage of their repressor, the lexA gene product. Cleavage in vivo requires functional RecA protein in a role not yet understood. We used mRNA hybridization techniques to follow the rapid changes that occur with induction in cells with mutations in the recA operator or in the repressor cleavage site. These mutations allowed us to uncouple the induction of RecA protein synthesis from its role in inducing the other SOS functions. Following induction with ultraviolet light, we observed increased rates of mRNA synthesis from five SOS genes within five minutes, maximum expression ten to 20 minutes later and then a later decline to near the initial rates. The presence of a recA operator mutation did not significantly influence these kinetics, whereas induction was fully blocked by an additional mutation in the repressor cleavage site. These experiments are consistent with activation of RecA protein preceding repressor cleavage and derepression of SOS genes. The results also suggest that the timing and extent of induction of individual SOS genes may be different.