The genetics of ANT(2") and AAC(3)-II mediated gentamicin resistance in Gram-negative bacteria obtained from Hacettepe University Hospital in Turkey, was studied. The plasmid DNA analyzed by agarose gel electrophoresis revealed a heterogeneous plasmid population. Conjugal transfer appeared to be an efficient mechanism for the dissemination of resistance to gentamicin/tobramycin/kanamycin/streptomycin/chloramphenicol/cephalothin , which was the common resistance pattern in the population. Plasmids of 46, 60, 70 and 80 Mdal in size were found to be conjugative. Among these a 70 Mdal plasmid was isolated more frequently; it was harboured by the strains exhibiting either of the enzyme profiles, ANT(2") or AAC(3)-II. Like the substrate profiles of the donors, their enzymatic activities were also maintained in the respective transconjugants. Based on the donor and transconjugant resistance markers, agarose gel electrophoresis and transfer properties, ANT(2") and AAC(3)-II dependent gentamicin resistance was found to be plasmid-linked. However, the evidence of a possible chromosomal location for the gentamicin resistance suggested that the resistance might well be of a transposable nature.