The rexA and rexB genes of bacteriophage lambda are expressed from the prophage and cause the exclusion of many superinfecting mutant phages. We cloned the rexA and rexB genes into a multicopy plasmid so that they were overexpressed from the inducible tac promoter. No obvious phenotypes were associated with overexpressing both rexA and rexB or overexpressing rexA in the absence of rexB expression. However, induction of rexA in the presence of limiting rexB activity caused an immediate cessation of cell growth. All macromolecular synthesis abruptly ceased and amino acid transport was severely inhibited. Intracellular levels of adenosine 5'-triphosphate also dropped. These phenotypes are similar to those observed after phage superinfection, leading us to propose that at least some of the exclusion caused by the Rex proteins could be due to a change in their ratio following superinfection.