Amdinocillin has been shown to have broad coverage in vitro against many strains of Enterobacteriaceae. Synergy has been demonstrated in vitro with several other beta-lactam antibiotics. The rabbit model of meningitis was used to study the in vivo effectiveness of amdinocillin when combined with other beta-lactams for serious infections. Organisms that showed an enhanced in vitro bactericidal effect from the combination of amdinocillin and another beta-lactam showed more rapid elimination of the organisms in vivo.