A strain of Acinetobacter baumannii 6B92 isolated from the blood culture of a patient at the Edinburgh Royal Infirmary i 1985 was found to be resistant to imipenem, all classes of cephalosporins and penicillins. Extraction of the soluble proteins of the cell and isoelectric focusing revealed the presence of two beta-lactamases: a chromosomal cephalosporinase of high pI (> pI 9.0) and a novel beta-lactomase of pI 6.65 named ARI 1 (Acinetobacter resistant to imipenem). Despite the fact that original clinical isolate could be 'cured' of its resistance to imipenem and penicillins by growing in the presence of ethidium bromide with the concurrent loss of the ARI 1 enzymes, no resistance plasmid was visualised or transferred. The ARI 1 beta-lactamase hydrolysed penicillin, ampicillin and cephaloridine slowly during enzyme assay but inactivation of imipenem could only be demonstrated by microbiological means. The molecular size of the ARI 1 enzyme was 23 kDa and it was not inhibited by EDTA, p-CMB, or clavulanate.