Abstract: Objective: To investigate the production of β-lactamases in enterobacteriaceae from lower respiratory tract and measure the antimicrobial activities in vitro of several antibiotics to enzyme-producing strains.Methods: Cefoxitin three-dimensional test and disk diffusion confirmation method recommended by NCCLS were used to detect AmpC enzyme and ESBLs respectively. Antimicrobial activity in vitro and sensitivity of drugs were measured by twofold agar dilution method and kirby-bauer agar diffusion method respectively. Results: Among 80 clinical isolates there were 39 strains enterobacter cloacae, 25.6% of those produced ESBLs, 41.0% for AmpC enzyme and 7.7% for ESBLs+AmpC;then followed by klebsiella pneumoniae and escherichia coli. In seven fluoroquinolones resistance rates of ciprofloxacin, norfloxacin, ofloxacin and pefloxacin were higher than that of moxifloxacin, gatifloxacin and levofloxacin, the former was over 50% and the latter was in 30%-40% to enzyme-producing strains. In other four common antibiotics resistance rate of imipenem was lower, 16.7%, 9.5% and 0 to ESBLs, AmpC and ESBLs+AmpC strains respectively. Resistance rate of amikacin was in 30%-40%, and that of cefotaxime and ceftazidime were highest, over 70%. Conclusions: Enterobacter cloacae was number one and had the highest anzyme-producing rate in clinical specimens from lower respiratory tract. Imipenem had higher antimicrobial activity; the next were moxifloxacin, gatifloxacin, levofloxacin and amikacin in several antibacterial drugs.