A clinical isolate of Escherichia coli, strain CB-134, recovered in 1986 from an abdominal abscess, exhibited resistance to penams, oxyimino-beta-lactams including broad-spectrum cephalosporins (cefotaxime, ceftriaxone, ceftazidime), and aztreonam but remained susceptible to cephamycins (cefoxitin, cefotetan) and to moxalactam and imipenem. Clavulanate (2 micrograms/ml) restored the susceptibility of the strain to broad-spectrum cephalosporins and aztreonam. A beta-lactamase with an isoelectric point (pI) of 5.9 was detected in strain CB-134, and the corresponding gene was transferred by conjugation to E. coli together with the associated aminoglycoside resistance determinant [AAC(3)-II] and tetracycline, trimethoprim, and sulfonamide resistance. The beta-lactamase efficiently hydrolyzed cefotaxime and ceftriaxone but only moderately hydrolyzed ceftazidime and was inhibited by clavulanate and sulbactam (1 microM) and by anti-TEM-1 and anti-TEM-2 sera. This extended-spectrum beta-lactamase, conferring resistance to cefotaxime, ceftriaxone, ceftazidime, and aztreonam, was comparable to CTX-1 (TEM-3) but differed from it by pI. Agarose gel electrophoresis of the plasmid DNA indicated that this new enzyme was coded by pUD16, a plasmid of 220 kilobases which belongs to the Inc6 incompatibility group. Hybridization with an intragenic probe for TEM-1 revealed that this beta-lactamase derives from TEM-type beta-lactamases and hence it was named TEM-4.