ABSTRACTOxazolidinones are efficacious in treating mycobacterial infections, including tuberculosis (TB) caused by drug-resistant Mycobacterium tuberculosis. In this study, we compared the in vitroactivities and MIC distributions of delpazolid, a novel oxazolidinone, and linezolid against multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) in China. Additionally, genetic mutations in 23S rRNA, rplC, and rplDgenes were analyzed to reveal potential mechanisms underlying the observed oxazolidinone resistance. A total of 240 M. tuberculosisisolates were included in this study, including 120 MDR-TB isolates and 120 XDR-TB isolates. Overall, linezolid and delpazolid MIC90values for M. tuberculosisisolates were 0.25 mg/liter and 0.5 mg/liter, respectively. Based on visual inspection, we tentatively set epidemiological cutoff (ECOFF) values for MIC determinations for linezolid and delpazolid at 1.0 mg/liter and 2.0 mg/liter, respectively. Although no significant difference in resistance rates was observed between linezolid and delpazolid among XDR-TB isolates (P> 0.05), statistical analysis revealed a significantly greater proportion of linezolid-resistant isolates than delpazolid-resistant isolates within the MDR-TB group (P= 0.036). Seven (53.85%) of 13 linezolid-resistant isolates were found to harbor mutations within the three target genes. Additionally, 1 isolate exhibited an amino acid substitution (Arg126His) within the protein encoded by rplDthat contributed to high-level resistance to linezolid (MIC of >16 mg/liter), compared to a delpazolid MIC of 0.25. In conclusion, in vitrosusceptibility testing revealed that delpazolid antibacterial activity was comparable to that of linezolid. A novel mutation within rplDthat endowed M. tuberculosiswith linezolid, but not delpazolid, resistance was identified.