The gene encoding the metallo-beta-lactamase SPM-1 was cloned from a genomic library of Pseudomonas aeruginosa strain 48-1997 A. The insert carrying spm-1 possessed a GC content of 47%, indicating that it is of non-Pseudomonas origin. Upstream of spm-1 there is a small open reading frame (ORF), which is homologous to the LysR family of proteins (69% identity to the LysR protein from Salmonella enterica serovar Typhimurium). Downstream of spm-1 there is the start of an ORF, the product of which shows close homology with the GroEL-type proteins from Xanthomonas campestris. No transmissible element could be identified upstream or downstream of spm-1. The spm-1 gene is carried on a plasmid that can transform both Escherichia coli and P. aeruginosa to ceftazidime resistance. SPM-1 contains the classic metallo-beta-lactamase zinc-binding motif HXHXD and shows the highest identity (35.5%) to IMP-1. SPM-1 is a distinctly different metallo-beta-lactamase from VIM and IMP and, accordingly, represents a new subfamily of mobile metallo-beta-lactamases. The predicted molecular weight of the protein was 27 515 Da, significantly higher than that of IMP (25 041 Da) or VIM (25 322 Da). SPM-1 possesses a unique loop of 23 residues that accounts for the higher molecular mass.