The carbapenems, imipenem and meropenem, are anomalous β-lactams in their antipseudomonal behaviour, as well as in their β-lactamase stability. In the case of imipenem, MICs for Pseudomonas aeruginosa isolates are unrelated to those of other β-lactams, whereas a strong relationship exists between the MICs of different penicillins and cephalosporins for most Pseudomonas isolates.1–4 This observation is partly because imipenem MICs are independent of derepression of the chromosomal AmpC β-lactamase, whereas this factor strongly co-determines the MICs of penicillins and cephalosporins.5 More critically, the MICs of imipenem are unaffected by the broad-spectrum intrinsic resistance expressed by many P. aeruginosa isolates, whereas this mechanism strongly co-determines the MICs of penicillins, cephalosporins and several unrelated drug classes, including quinolones, tetracyclines and chloramphenicol.1–4