One hundred and ninety-five multi-resistant strains of Klebsiella pneumoniae were isolated at Princess Alexandra Hospital (PAH) between December 1991 and June 1995. All these organisms produced extended spectrum β-lactamases (ESBLs) as detected by the double disc synergy test (DDST). Between June 1994 and June 1995, a second population of 67 multi-resistant but DDST negative strains was isolated. Twenty multi-resistant Klebsiella pneumoniae strains 116 DDST positive and four DDST negative) and one susceptible strain were selected for further study. These were tested for production of ESBLs by two double disc synergy methods and agar dilution minimum inhibitory concentrations (MICs) with and without clavulanic acid. Detected ESBLs were further characterised by isoelectric focusing. The confirmed DDST positive K. pneumoniae strains all produced ESBLs that focused at an isoelectric point (pl) of 7.6, suggesting the presence of SHV-2, SHV-2a, SHV-6, SHV-7 or SHV-8 enzymes. The multi-resistant DDST negative strains showed no clavulanic acid synergy and thus no evidence of the presence of ESBLs.