Administration of either gentamicin or amikacin induced an increase in the number of amikacin-resistant (AR) isolates of certain Enterobacteriaceae and Acinetobacter species in a hospital in Buenos Aires. A total of 127 AR isolates was selected to study the molecular mechanisms of resistance involved. The aac(6')-Ic gene was found by dot-blot hybridisation in every Serratia marcescens isolate. A gene different from aac(6')-Ia, aac(6')-Ib and aac(6')-Ic encoding the AAC(6')-I activity was found in a 15.5-kb plasmid in Acinetobacter spp. Plasmids from 27 Enterobacteriaceae contained and aac(6')-Ib gene and 26 of these carried sequences related to the Tn1331 transposon, whereas one Escherichia coli plasmid showed homology in another fragment of the Tn1331 transposase. Because plasmids bearing the aac(6')-Ib gene were heterogeneous, dissemination of the aac(6')-Ib gene may have been due to transposition of Tn1331 rather than the spread of an epidemic plasmid. The rate of AR isolates varied within each species in spite of the presence of Tn1331, and it is likely, therefore, that this transposon may not be the sole factor responsible for the observed variation. The aph(3')-VIa gene (originally described in Acinetobacter spp.) was found with high frequency (80%) in this Acinetobacter population. Furthermore, this gene was found also in plasmids from 20% of other gram-negative organisms commonly involved in nosocomial infections in this hospital.