Virtually all (94.5 per cent of 110) chloramphenicol-resistant strains of Sbigella, Escherichia , and Staphylococcus isolated from clinical cases caused significant inactivation of the antibiotic, but none of the 29 resistant pseudomonads did so. None of the 235 clinically isolated sensitive strains, representing five genera, inactivated the drug. Resistant organisms derived in vitro from initially sensitive ones by repeated subculture in increasing concentrations of chloramphenicol had only limited ability to inactivate the antibiotic, but those produced via transformation inactivated it almost completely. Such transferred resistance was lost following treatment with acriflavine. Simultaneously, ability to inactivate chloramphenicol was lost.