The Third generation
cephalosporins is one of the widely used antibiotics in clinic. Since 1983, many resistant organisms
isolated from clinic were be found that the resistance is due to new enzymes-extended spectrum β-lactamaces(ESBLs). In our present paper, 78 strains resistant to cefoperazone, cefotaxime, ceftriaxone and ceftazidime were isolated from 866 clinical pathogens in Beijing district. 4 isolates were chosen to be further studied with respect to their β-
lactamases and genetic background. Two β-lactamases were purified 105.93 and 244.22 folds respectively by using FPLC. The study showed that 4 β-lactamases had a strong hydrolyzing activity against cefotaxime, ceftriaxone and cefoperazone but not to ceftazidime. The isoelectric points of these β-lactamases were >7, This suggested that they were not derived from TEM family. They could be inhibited by suicide β-lactamase inhibitor and were class A enzyme. The study of transformation and Southern hybridization showed these β-lactamases were mediated by plasmid and not belong to the SHV family. These β-lactamases may belong to the new group of non TEM or SHV derived β-lactamases..