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The maintenance and stability in an E. coli K12 host of environmental isolates of R plasmids encoding gentamicin resistance during multiple passages in antibiotic-free and antibiotic-containing broth were investigated in regard to the conferred resistance phenotypes and the respective EcoRI digestion patterns. Only two plasmids belonging to the IncM group maintained stable endonuclease digestion patterns over a 15-month period in both of the media applied. Other members of this group revealed a considerable variability in their EcoRI digestion patterns, but a stability in their resistance determinants. Two IncM plasmids and an IncK plasmid exhibited partial resistance loss under nonselective conditions. The complete segregation of resistance determinants from IncOF plasmids resulted in a stably maintained cryptic core plasmid.