A multiple antibiotic‐resistant Staphylococcus aureus, was found to possess three plasmid bands in agarose gel electrophoresis. A plasmid of approximately 4.3 kb (pMC790/2) was found to code for ampicillin and tetracycline resistance and to have one EcoRI site when transformed into S. aureus RN 4220. pMC790/2 in unmodified form was transformed into a recA−E. coli at a frequency of 1.2×104 transformants/μg of plasmid DNA. Plasmid (pMC790/2) replicated, maintained itself stably and expressed far better in the E. coli host than in S. aureus.