Compared to the first and second-generation cephalosporins, the third-generation cephalosporins are remarkably stable against hydrolysis by the $eta$-lactamases produced by aerobic gram-negative bacilli, such as Enterobacteriaceae. Among these bacteria, the most prevalent plasmid-encoded $eta$-lactamase is TEM-1 $eta$-lactamase belonging to class A or group 2b. This enzyme is produced constitutively and is principally active against peniciflins and old cephalosporins rather than third-generafion cephalosporins, carbapenems and mmobactams. However, new TEM type $eta$-lactamases including TEM-9 and TEM-12 evolved through point mutations in a gene encoding $eta$-lactamase have been discovered from patients during chemotherapy. These $eta$-lactamases are known to be capable of hydrolyzing most of the third-generatim cephalosporins. To study the prevalence of $eta$-lactamases from clinical isolates collected in Korea. the minimal inhibitory concentratims(MICs) of several third-generation cephalosporins against 628 clinical isolates were determined by agar dilution methods, and $eta$-lactamas-producing bacteria were isolated by use of cefinase disc. By polymerase chain reaction (PCR) method, clinical isolates harboring a gene for TEM type $eta$-lactamase were identified among the $eta$-lactamase producing strains. Twentiy three percent of the clinical isolates was resistant to the thirdgeneration cephalosporins, and more than 90% of resistant cells produced various $eta$-lactamases. TFM type $eta$-lactamases were dominant in gram-negative bacilli, such as Escherichia coli, Klebsiella pneumoniae, Enterobacter species. These results suggest the necessity of the development of new cephalosporins which are stable against $eta$-lactamases like TEM.