CW-270031, an injectable carbapenem, is a novel synthesized pyrrolidinyl-thio carbapenems. It was evaluated for its in vitro antibacterial activities in comparison with those of imipenem and meropenem against standard strains and clinical isolated strains, CW-270031 was more active than imipenem against gram-negative (E. coli and Klebsiella oxytoca) clinical isolates, but it was slightly active than meropenem. Against Klebsiella aeruginosa CW-118 MIC were 0.048 $mu$g/ml for CW-270031, 0.19 $mu$g/ml for imipenem. Against clinical E. coli MIC range were 0.012$sim$0.195 $mu$g/ml for CW-270031, 0.097$sim$0.39 $mu$g/ml for imipenem. Against clinical Klebsiella oxytoca MIC$_{50}$ were 0.09 $mu$g/ml for CW-270031, 0.39 $mu$g/ml for imipenem. Against gram-positive standard strains and clinical CW-270031 was slightly more activity than meropenem, but CW-270033 was less active than imipenem against these tested isolates. The subcutaneous injection of CW-270031 in mice revealed that the half-life of CW-270031 in serum was about 13 min, long than that of meropenem (10.6 min). CW-270031. was stable to hydrolysis by dog renal dehydropeptidase I (DHP-l) enzyme, to an more stabilities shown by meropenem.