The effect of cerulenin on the production of beta-lactamase and other periplasmic proteins was studied in Escherichia coli IA199 carrying plasmid pBR322. Cerulenin (10 to 25 microgram/ml) had almost no effect on the growth rate of E. coli but it decreased the amount of beta-lactamase and other periplasmic proteins in shock fluid. Higher amounts of the antibiotic (40 to 100 micrograms/ml) decreased turbidity and almost completely prevented synthesis of beta-lactamase and other periplasmic proteins. Cerulenin decreased incorporation of L-[35S]methionine into membranes during growth as well. Spheroplasts secreted beta-lactamase into the external medium, but during a 3-h incubation in the presence of cerulenin (25 micrograms/ml) this secretion was prevented by more than 90%. Beta-Lactamase was secreted into the isolated membrane vesicles from E. coli IA199. However, only 5% of the total amount of pre-beta-lactamase was secreted and processed by the membranes in vitro. Cerulenin did not prevent processing in vitro but the membranes prepared from the cells grown in the presence of cerulenin (25 micrograms/ml) did not catalyze processing of pre-beta-lactamase at all. Membrane preparations from Bacillus subtilis did not process pre-beta-lactamase either in the absence or in the presence of cerulenin.