In an international collection of 108 epidemiologically distinct strains of gentamicin-resistant Klebsiellae, extensive plasmid-mediated multiple antibiotic resistance was found in 80.6 percent of strains. In vivo conjugation was recognized in eight patients within two years in one hospital, and identical plasmids were found in different serotypes from hospitals in West Germany, the United Kingdom, Italy and Australia. Of 45 patients who were infected or colonized with gentamicin-resistant Klebsiellae, 26 had positive skin sites and of these, six had negative clinical specimens and served as a "hidden" reservoir of strains which provide a source for the contamination of nurses' hands and sustain nosocomial outbreaks. It is suggested that the few agents effective against gentamicin-resistant Klebsiellae, such as amikacin and cefuroxime, should be reserved; that gentamicin-resistant strains are selectively favored by the use of agents other than gentamicin which are more widely prescribed; and that more careful identification of skin carriers and attention to staff handwashing will control the spread of such strains and their plasmids.