Pseudomonas aeruginosa AH, isolated in Ankara, Turkey, was highly resistant to ceftazidime (MIC, 128 microg/ml) and produced a beta-lactamase that gave a doublet of bands at pIs 8.7 and 8.9. beta-Lactamase production was transferable to P. aeruginosa PU21 by conjugation and was determined by a ca. 450-kb plasmid, pMLH54. The transconjugant and Escherichia coli transformed with the cloned gene showed increased resistance to ceftazidime (especially) and to cefpirome, ceftazidime, ceftriaxone, moxalactam, and aztreonam, but not to carbapenems. Resistance was not reversed by clavulanic acid or tazobactam. Sequencing revealed that the beta-lactamase responsible for this resistance was identical to OXA-2 except that glycine replaced aspartate at position 150. Compared to OXA-2, the new enzyme, named OXA-15, had greater cephalosporinase activity, with increased relative hydrolysis rates for cephaloridine and cephalothin and, most dramatically, for ceftazidime. Cefotaxime and carbapenems remained stable to hydrolysis. Thus, as in the TEM, SHV, and OXA-10 (PSE-2) beta-lactamase families, a minor sequence change in OXA-2 gave a major extension of cephalosporinase activity and contingent resistance. The gene encoding the new beta-lactamase, bla(OXA-15), lay close to the highly conserved 3' end of an integron and had flanking sequences typical of an integron-associated gene cassette. Restriction mapping and partial sequence data indicated that pMLH54 carries an integron with three putative gene cassettes: bla(OXA-15) itself, aadB [coding aminoglycoside nucleotidyltransferase (2")-1a], and an uncharacterized cassette.