Penicillin-binding proteins (PBPs) were visualized in strains of Escherichia coli that carried mutations in one or more of the following protease genes: tsp, degP, ptr, and ompT. In the absence of a functional ompT gene, PBPs 1b alpha and 7 were not processed to the shortened forms 1b beta and 8, respectively. Cleavage of PBPs 1b alpha and 7 could be restored by introduction of a plasmid carrying the wild-type ompT gene. These PBPs were processed only after cell lysis or after membrane perturbation of whole cells by freeze-thaw, suggesting that the cleavage was a nonspecific artifact due to contact with OmpT, an outer membrane protease, and that such processing was not biologically significant in vivo. The degradation of other PBPs during purification or storage may also be effected by OmpT.