Methicillin- and gentamicin-resistant Staphylococcus aureus (MGRSA) strains isolated from Dublin Hospitals were classified into two groups (phenotypes). Phenotype-I strains expressed high level resistance to gentamicin and were susceptible to fusidic acid; strains resistant to tetracycline harboured a 3 X 10(6)-mol. wt plasmid. Strains in phenotype II usually expressed low level resistance to gentamicin, were resistant to fusidic acid and often harboured a (22-24) X 10(6)-mol. wt plasmid that specified resistance to ethidium bromide, tetracycline, kanamycin, neomycin and trimethoprim, or to combinations of these markers. A few phenotype-II strains expressed higher levels of resistance to gentamicin and other aminoglycosides. All MGRSA strains carried a 21 X 10(6)-mol. wt plasmid conferring resistance to penicillin, ethidium bromide, cadmium and mercury. Gentamicin resistance was invariably chromosomal and all strains carried chromosomal resistance to methicillin, erythromycin, streptomycin and spectinomycin. Several methicillin-resistant S. aureus (MRSA) strains isolated before the emergence of gentamicin resistance harboured a 21 X 10(6)-mol. wt penicillinase plasmid with the same restriction endonuclease profile as that from some MGRSA strains. Some MRSA strains carried other plasmids related to those found in MGRSA strains.