Resistance to chloramphenicol [56-75-7] in a strain of H. parainfluenzae was due to the presence of a drug-inactivating system within the organism.Chloramphenicol inactivation was apparently caused by acetylation of the mol., since incubation of the resistant strain with chloramphenicol and acetyl CoA yielded a product that moved on thin-layer chromatog. the same as chloramphenicol acetate standardsChloramphenicol resistance may be plasmid borne, this being the case in some other gram-neg. bacteria resistant to chloramphenicol.