To overcome the problems of the resistance to clavulanic acid, many researchers are developing novel inhibitors that are not sensitive to new mutant ${eta}$-lactamases. In order to evaluate newly synthesized compound CH2150 (Sodium (3S.5R)-6(Z)-[1-{1-(2-{2-benzoxazoly}thioethyl)-l.2,3-txiazol-4-yl}methylene] penicillanate-1,1-dioxide) as a ${eta}$-lactamase inhibitor, we examined inhibitory activity of CH2150 against ${eta}$-lactamases of clinical isolates resistant to ampicillin/sulbactam(12 strains of Escherichia coli and 13 strains of Staphylococcus aureus), and compared with that of sulbactam. Nitrocefin was used as substrate for ${eta}$-lactamases, and the increase of absorbance was measured spectrophotometerically at 482 nm. ${eta}$-Lactarnase inhibition of CH2150 against ${eta}$-lactamases was 73 ~ 96% in E. coli and 76 ~ 79% in S. aureus. Comparatively, that of sulbactam was 96 ~ 100% and 96 ~ 100%, respectively. The inhibitory activity of CH2150 was slightly lower than that of sulbactam. The MIC values of ampicillin combined with CH2150 (2:1) for the clinical isolates were 4~512 ${mu}$g/ml for E. coli and 1.0 ~ 64 ${mu}$g/ml for S. aureus, whereas 0.5~16 ${mu}$g/ml for E. coli and 0.25~8 ${mu}$g/ml for S. aureus when combined with sulbactam (2:1).