Aminoglycoside resistance mechanisms from recent studies were compared with those found in earlier studies in the USA and Europe for three pathogen groups. Among Citrobacter-Enterobacter-Klebsiella, four single mechanisms (AAc(3)-II, AAC(3)-I, ANT(2")-I and AAC(6')-I were found in all studies, but the most recent studies showed a significant increase in combinations of AAC(6')-I with the other common mechanisms. Since AAC(6')-I confers resistance to tobramycin, netilmicin and amikacin, combinations of it with the other gentamicin modifying enzymes conferred broad-spectrum resistance to all clinically available aminoglycosides except isepamicin. Similar changes occurred in Escherichia-Morganella-Proteus-Salmonella-Shigella except that the frequency of combinations was much lower and two additional single mechanisms - AAC(3)-IV and permeability - were also found frequently. Among aminoglycoside-resistant Pseudomonas, three mechanisms, AAC(6')-II, ANT(2")-I and permeability, were always common and remained common. However, combinations of the three mechanisms with each other and with other mechanisms were more common in the recent surveys. Different genes which produce different proteins with the same aminoglycoside-modifying activity are now known. The results of hybridisation studies with two aac(3)-I, 2 aac(6')-II and 4 aac(6')-I gene probes are presented. The most commonly occurring genes were: aac(3)-Ia, aac(3)-IIa, aac(6')-IIa, aac(6')-Ib and, in Serratia, aac(6')-Ic. The activity of isepamicin against amikacin resistant strain which produce AAC(6')-I can be related to differences in the structure of these two similar aminoglycosides at Position 3". Amikacin may form a stable complex with AAC(6')-I enzymes via binding interaction at Position 3 and 3". Isepamicin, which has a secondary amino group at Position 3", may only be able to interact at Position 3 and enzyme-isepamicin complexes are likely to be less stable.