“…It is generally recognized that most of these plasmids con fer to the bacteria the ability of destroying or inactivating antibiotics [6], For chloram phenicol, it has been shown that wild type strains of resistant Escherichia coli carrying R factors are able to synthetize a specific acetylase inactivating the drug [3,4,7]; the antibiotic is acetylated mainly in position 1 and 3 (1,3-diacetoxy-dcrivative) or in posi tion 3 (3-monoacetoxy-derivative), and, to a lower extent, in position 1 (1-monoacetoxyderivative) [4,8], Chloramphenicol binds to the 50 S subunits of the ribosome, probably inhibiting peptide bond formation [2,11,13], Preliminary investigations on the effect a wild type Escherichia coli strain carrying None of the acetylated derivatives has been of the 3-monoacetoxy-derivative and the 1,3-diacetoxy-derivative showed that the latter compound has no activity in vivo or in vitro, whereas the former has little inhibi tory.activity [5].…”