By means of restriction endonuclease digests and DNA/hybridisation studies we analysed ten representative methicillin-resistant Staphylococcus aureus strains of our collection for plasmid similarities and plasmid associated resistance determinants. We found that strains isolated at our laboratory contained identical or at least most similar plasmids. Isolates from another geographical origin showed different plasmid patterns. We found resistance determinants for gentamicin to be chromosomally encoded, whereas resistance to heavy metal ions and chloramphenicol was always plasmid associated. Resistance to trimethoprim, tetracycline and erythromycin was usually chromosomally mediated but could also reside on a plasmid. Our results indicate that methicillin-resistant strains from our collection may have a common origin. The clinical relevance of these results is discussed.