Staphylococcus epidermidis clinical isolate MH6502 contained the 51. 9-kb nonconjugal plasmid pMH6502, which has homology to a major part of the transfer gene region of a known conjugal plasmid. Plasmid pMH6502 mediates aminoglycoside and ethidium bromide resistance. During restriction digest analysis of pMH6502, a double logarithmic regression of marker data gave a better linear relationship than a semi-logarithmic one. The analysis indicated several differences in the transfer-like region of pMH6502 compared to the analogous region of the S. aureus conjugal plasmid pG01. The transfer-like region was in the opposite orientation compared to pG01. An EcoRI site that is within the transfer-like region of pMH6502, has no analogue in pG01. A HindIII site, located outside a 6.3-kb EcoRI fragment in the transfer gene region of pG01, is inside the analogous fragment of pMH6502. A model is proposed to describe how a conjugal ancestral plasmid of pMH6502 could alter to its present form.