A large number of Staphylococcus aureus strains resistant to gentamicin, methicillin and other antibiotics, isolated in several Dublin hospitals during a 4-year period, were screened for plasmid DNA. Isolates assigned to four principal phage groups showed uniform plasmid profiles. A plasmid of mol. wt 21.0 x 106 encoding penicillin resistance was present in all isolates screened. A tetracycline resistance plasmid of mol. wt 24.0 x 106 was present in 40% of isolates of phage types 90 and 5/47/54/84/85 whereas a plasmid of mol. wt 3.0 x 106 was responsible for tetracycline resistance in isolates of phage types 77 and 85. Aminoglycoside, methicillin and erythromycin resistance were apparently not encoded on plasmids. Gentamicin resistance was transduced at low frequency between several strains of S. aureus and the resistance phenotype was due, in part at least, to the production of an inactivating enzyme.