Until 1983, third generation cephalosporins (3GC) were thought to be resistant to hydrolysis by all plasmid-mediated β-lactamases. However, TEM and SHV derived β-lactamases have recently evolved which can confer transferable resistance to 3GC. Six novel plasmid β-lactamases, which confer transferable resistance to 3GC have been identified and characterised. They have been compared directly to the other 3GC hydrolysing β-lactamases discovered elsewhere. The criteria implemented to distinguish these different β-lactamases were: Km and Vmax values for the hydrolysis of different substrates, molecular weight, isoelectric focusing point (pi) and susceptibility to inhibitors. The β-lactamases TEM-E1, TEM-E2, TEM-E3, and TEM-E4, were all TEM derived and, although they conferred a greater transferable resistance to ceftazidime than cefotaxime, they hydrolysed both these substrates with similar efficiencies. TEM-E2 was produced by an organism which was isolated in Liverpool in 1982 and is, consequently, the first example of transferable 3GC resistance. TEM-E3 was produced by clinical isolates from two different London hospitals, and found to be the same as IBM-10 which was discovered in the USA after the characterisation of TEM-E3. When ceftazidime was used as a selecting agent, TEM-E1 and TEM-E2, could be obtained spontaneously from a TEM-1 producing E.coli and, in the same way, TEM-E4 could be obtained from a TEM-2 producing organism. Utilising the same methodology a β-lactamase, which conferred transferable resistance to ceftazidime, was obtained from PSE-4, although such an enzyme has not yet been reported in clinical isolates. The fifth novel β-lactamase, E8825 (pi 7.9), was produced by an organism isolated in India which also produced TEM-1 and CAZ-6. This was the first example of transferable 3GC resistance in Asia and the first report of two transferable 3GC hydrolysing β-lactamases being encoded by the same plasmid. The characteristics of the sixth novel enzyme, DJP-1, suggest that it was originally an E.coli chromosomal β-lactamase. Analysis of this strain revealed that the β-lactamase (pi greater than 8.2) conferred transferable resistance to all β-lactam antibiotics and clavulanic acid. Finally, two novel methods for the isolation and purification of (3-lactamases were developed during these studies. Electro-dialysis of 0-lactamases from polyacrylamide gel allowed rapid purification of TEM-E2 from TEM-1. Fast Liquid Protein Chromatography (FPLC System) was employed to separate β-lactamase E882S from the other β-lactamases produced by the host strain. In addition it was illustrated that different β-lactam molecules can induce a variety of different β-lactamase satellite bands which can be visualized by isoelectric focusing. Moreover, it was also verified that these satellite bands were variants of the main β-lactamase protein.