Clinically isolated bacterial strains resistant to almost of all the clinically superior .betha.-lactam antibiotics can be used to screen the promising ones among the newly synthesized $eta$-lactam antibiotics. To select the resistant strains, the susceptibility of 389 strains of S. aureus, 144 strains of coagulase negative staphylococci, 509 strains of E. coli, 115 strains of E. cloacae and 187 strains of P. aeruginosa to methicillin, ampicillin, piperacillin and gentamicin was determined. The susceptibility of 19 bacterial strains selected through the first screening to cefixime, cefotiam, cefotaxime, flomoxef, cepfirome, cefdnir, SCE-2787, panipenem and imipenem was determined. Four strains of S. aureus finally selected have high degree of resistance to almost of all $eta$-lactam antibiotics used and also produce $eta$-lactamases. These 4 strains of S. aurues can be used to screen effectively the promising $eta$-lactam antibiotics among the numerous numbers of the newly synthesized $eta$-lactam antibiotics.