Controlled experiments were conducted to determine the in-vitro postantibiotic effect (PAE) of meropenem and imipenem on ten selected bacteria representative of medically important species: Staphylococcus aureus (2). Pseudomonas aeruginosa (4), Escherichia coli (1), Serratia marcescens (1), Morganella morganii (1), and Providencia stuartii (1). The PAE was determined by comparing serial colony counts of cultures recovering from exposure to drug concentrations at 4 x MIC for 1.5 h with the counts of drug-free controls. A PAE was observed with both meropenem and imipenem when tested against four strains of Ps. aeruginosa (meropenem PAE = 0.8-2 h; imipenem PAE = 1.2-2.5 h) and two strains of Staph. aureus (meropenem PAE = 0.7, 1.7 h; imipenem PAE = 1.7, 1.8 h). In addition, a PAE was observed with meropenem on two of four Enterobacteriaceae, E. coli ATCC 25922 (0.8 h) and Prov. stuartii (1.2 h), but not with one strain each of M. morganii and Ser. marcescens. A PAE was not observed when imipenem was tested against the four Enterobacteriaceae. Studies are suggested to investigate further the PAE of meropenem on additional strains of Enterobacteriaceae.