A clinical isolate of Serratia marcescens (TN9106) produced a metallo 13-lactamase (IMP-1) which conferred resistance to imipenem and broad-spectrum 13-lactams. The consensus amino acid residues, His-95, His-97, Cys-176, and His-215, which form putative zinc ligands, were conserved in the deduced amino acid sequence of IMP-1. By determination of the amino acid sequence at the N terminus of purified mature IMP-1, 18 amino acid residues were found to be processed from the N terminus of the premature enzyme as a signal peptide. These results clearly show that IMP-1 is an enterobacterial metallo 13-lactamase, of which the primary structure has been completely determined, that confers resistance to carbapenems and other broad-spectrum 13-lactams.Many extended-spectrum 13-lactamases conferring high levels of resistance to broad-spectrum P-lactam antibiotics have been found worldwide with the increasing use of newly developed broad-spectrum P-lactam antibiotics (10, 11).They are mostly R plasmid-mediated TEM-or SHV-related class A enzymes (4,12,18,22,29,32). However, plasmidmediated AmpC-type ,-lactamases that belong to class C have recently been reported (9, 21). These plasmid-mediated extended-spectrum ,B-lactamases have a serine residue at the active center of the enzyme, and they effectively hydrolyze broad-spectrum ,-lactams, except carbapenems. Although it was reported by genetic analyses that bacteria belonging to the family Enterobacteriaceae, except Klebsiella spp. (1, 2) and Pseudomonas spp., have chromosomally encoded inducible AmpC 3-lactamases (8,14,16,20,26,34) times show resistance to imipenem and broad-spectrum P-lactams (7,17,37). The imipenem resistances of these bacteria are mainly due to the production of metallo 3-lactamases that belong to class B. The class B enzymes require zinc ions for enzyme activity and demonstrate a primary structure quite different from those of the class A and class C enzymes belonging to the group of serine P-lactamases.Recently, several strains of Serratia marcescens were reported to show resistance to carbapenems as well (35). In this study, we clearly show that a clinically isolated strain of S. marcescens (TN9106) showing resistance to imipenem has a chromosomally encoded metallo 3-lactamase gene.