The frequency of selection of resistant variants by 10 different broad-spectrum β-lactam derivatives was evaluated for 10 clinical Enterobacter cloacae isolates. With respect to most penams or cephems, resistant variants could be selected up to 8- or 32-fold the MIC, respectively. However, with cefpirome as the selecting agent resistant variants were obtained only at twice the MIC, whereas resistant variants were barely detectable with temocillin and not detectable in any case with imipenem. The variants exhibited cross-resistance between penams and cephems including aztreonam, but not to temocillin and imipenem regardless of the β-lactamase amount produced. Enzyme production of the variants ranged from 0.2 U to 19.0 U β-lactamase/mg protein of the cell-free supernatants. Moreover, analysis of the outer membrane protein composition did not reveal marked alterations between wild strains and the corresponding variants. It is evident that ‘overproduction of the chromosomal β-lactamase’ cannot explain entirely phenotypic resistance to broad-spectrum β-lactam compounds, but a lack of porin production, i.e. major outer membrane proteins, cannot provide an explanation for the above