Background:Antimicrobial resistance is a serious problem that compromises the empirical treatment of infections,resulting in a lack of effective antibiotics and high medical expenses.Here,we aimed to monitor the trends in antimicrobial resistance among Enterobacteriaceae isolated from blood samples in mainland China.Methods:A total of 2240 Enterobacteriaceae isolates from blood were collected from hospitalized patients at 19 tertiary hospitals between October 2004 and June 2014.The minimum inhibitory concentrations of all isolates were determined using the agar dilution method according to the Clinical and Laboratory Standards Institute 2016 guidelines.Results:The most commonly isolated bacteria were Escherichia coli,compromising 47.0％ (1053/2240) of the total isolates,followed byKlebsiella spp.(26.3％),Sahnonella spp.(10.4％),and Enterobacter spp.(9.2％).The detection rates of extended-spectrum β-lactamases (ESBLs) among E.coli were 68.9％ (2004-2005),73.2％ (2007-2008),67.9％ (2009-2010),72.6％ (2011-2012),and 58.4％ (2013-2014),whereas those in ESBL-producing Klebsiellapneumoniae were slightly decreased (75.9％,50.0％,41.4％,40.2％,and 43.0％,respectively).Carbapenems were the most potent agents against the Enterobacteriaceae isolates,followed by moxalactam,tigecycline,and amikacin.However,there was a decrease in the susceptibility rates for carbapenems in all species,particularly K.pneumoniae (decreased by 10.6％ for imipenem) and Enterobacter aerogenes (decreased by 2 l.1％ for imipenem).Reviving antibiotics (tigecycline and polymyxins) showed good in vitro activity against Enterobacteriaceae.Conclusions:The activity of antibiotics against Enterobacteriaceae isolated from blood was decreased overall.Large proportions of ESBL-producing isolates were identified among E.coli and Klebsiella spp.Carbapenem-resistant isolates have become a major challenge in the treatment of infections.