During a six-month period in a hospital in Ireland, four patients were infected (isolation from blood cultures) and two were colonized (isolation from rectal swabs) with strains of Enterococcus faecium highly resistant to gentamicin. MICs of gentamicin were greater than 1000 mg/L for all six strains, and each possessed a plasmid of approximately 50 MDa. Resistance to gentamicin was transferable by conjugation from two of the six strains, and was associated with transfer of the 50 MDa plasmid. This plasmid hybridized with a DNA probe specific for the bifunctional AAC(6')-APH(2") aminoglycoside-modifying enzyme. The mechanism of high-level gentamicin resistance in these strains appeared therefore, to be identical to that encountered in Enterococcus faecalis strains worldwide and reported in E. faecium strains in the USA.