Inhibition of DNA synthesis in E. coli by treatment with carcinogenic and mutagenic agents results in the coordinate expression of a group of diverse functions (SOS functions) including lambda prophage induction, filamentous growth, and an error-prone DNA repair activity (SOS repair) believed to be responsible for ultraviolet mutagenesis. It has been proposed that this SOS induction proceeds via irreversible proteolytic inactivation of repressor(s) for SOS functions. To test this hypothesis, we investigated the effect of a protease inhibitor, antipain [(1-carboxy-2-phenylethyl)carbamoyl-L-arginyl-L-valylargininal], on SOS induction. We found that 0.5 mM antipain (which has no effect on cell growth, overall RNA and protein synthesis, or induction of beta-galactosidase) drastically decreases mutagenesis. Antipain also blocks expression of thermally induced mutator activity (another manifestation of SOS repair) and filamentous growth in a tif-1 mutant that expresses SOS functions at 42 degrees without inhibition of DNA synthesis or detectable DNA damage. Furthermore, antipain inhibits thermal induction of lambda prophage in the tif-1 mutant without affecting the kinetics of thermal induction of lambdacI857 prophage. This lambda mutant codes a temperature-sensitive repressor that is directly destroyed by heat and does not require the SOS induction pathway for inactivation at 42 degrees. From our results we conclude that antipain inhibits lambda prophage induction by blocking proteolytic inactivation of lambda repressor and that it inhibits the induction or expression of SOS repair and filamentous growth. Our results suggest a role for proteolytic cleavage in the regulation of SOS functions.