By comparison with other Gram-negative genera, it might be speculated that the main harbingers of beta-Lactam resistance in Acinetobacter spp. are beta-Lactamases that were probably introduced on bacterial plasmids. However, there are a number of Gram-negative bacteria that are able to either induce or produce chromosomal cephalosporinases constitutively. Perhaps surprisingly, some multiresistant strains were isolated from surgical patients before any obvious antibiotic therapy had been administered, and it appears that Acinetobacter is a genus which has a propensity to develop antibiotic resistance extremely rapidly, perhaps as a consequence of long-term evolutionary exposure of the genus to antibiotic-producing organisms in a soil environment. The contribution of these chromosomal ß-lactamase appears to be important in the expression of ß-lactam resistance, but probably works in concert with a permeability reduction and altered penicillin binding proteins which may already confer some inherent resistance.