SOS mutagenesis in prokaryotes is dependent upon the inducible activity of the chromosomally encoded UmuDC proteins, or homologous proteins such as MucAB or ImpCAB which are found on naturally occurring plasmids. Relative to Escherichia coli, however, Salmonella typhimurium is much less responsive to the mutagenic effects of DNA-damaging agents, despite the fact that it possesses both chromosomally and plasmid encoded umu-like operons. In E. coli, activation of the UmuD mutagenesis protein to UmuD' via RecA-mediated proteolysis is a critical step in the mutation fixation pathway. We have used a polyclonal antiserum raised against the E. coli UmuD and UmuD' proteins to show that S. typhimurium expresses cross-reacting material only after treatment with the DNA-damaging agent mitomycin C. The S. typhimurium umuDC operon, therefore, appears to be regulated by mechanisms similar to the E. coli umuDC operon. After induction, the S. typhimurium UmuD protein was processed to UmuD' in both S. typhimurium and E. coli. However, the S. typhimurium UmuD protein appears to be cleaved more efficiently than the E. coli UmuD protein under similar conditions. The data suggest that conversion of UmuD to the mutagenically active UmuD' is not the rate-limiting factor accounting for the weakly mutable phenotype of S. typhimurium.