The effect of conjugative plasmids of various incompatibility groups of the enterobacteria family on the activity of the cell restriction-modification system of type I (EcoK) was studied. Twenty-two conjugative plasmids of 15 incompatibility groups were tested. In addition to plasmids of the incI1 and incN groups studied earlier, conjugative plasmids of the incFII, incB/O, and incK groups were also shown to be able to weaken the action of type I restriction enzymes upon nonmodified DNA (Ard phenotype). A hybridization analysis of all the plasmid DNAs studied, using ard gene DNA sequences from the ColIb-P9 (incI1) plasmid as a probe, was performed. The ard locus of the R100 (incFII) plasmid was cloned in the pBR322 and pACYC184 vectors. The ard gene was located 2.5 kb from the oriT site in the leading region on the R100 conjugative plasmid.