Ten conjugative and two nonconjugative R plasmids, all of which were naturally isolated, were tested for their ability to confer epistatic susceptibility to kasugamycin upon the host Escherichia coli. All of the conjugative plasmids, which belonged to various incompatibility groups, conferred a plasmid-determined epistatic susceptibility to kasugamycin upon the host cells, whereas the nonconjugative plasmids did not.