Antibiotic sensitivity and resistance are often under the control of the bacterial chromosome. Frequently, however, an organism may exhibit resistance to one or several antibiotics as a dominant character determined by genes located on a plasmid, a relatively small, circular DNA molecule which replicates, with some degree of autonomy, in the bacterial cytoplasm. Such plasmids, termed drug-resistance (R) factors, generally also specify the formation of sex pili, filamentous appendages on the cell surface. These promote bacterial conjugation, and hence permit the transfer of a copy of the plasmid from the resistant organism to one which may previously have been drug-sensitive. Each ex-conjugant is then capable of acting as a plasmid donor during subsequent pairings, so that R factors are commonly responsible for the epidemic spread of multiple drug-resistance throughout an entire bacterial population. This can present serious problems in antibiotic therapy, particularly as plasmids are often transmissible between organisms of different species, and even different genera. The molecular nature, classification and behaviour of R factors is discussed.