Mycolic acid-containing bacteria inactivate rifampicin in a variety of ways such as glucosylation, ribosylation, phosphorylation and decolorization. These inactivations were found to be a species-specific phenomena in Nocardia and related taxa. Gordona, Tsukamurella and fast-growing Mycobacterium modified rifampicin by ribosylation of the 23-OH group of the antibiotic. Such ribosylation was not observed in Rhodococcus and Corynebacterium, but phosphorylation of the 21-OH group of rifampicin was observed in one strain of Rhodococcus. Nocardia modified the antibiotic by glucosylation (23-OH group) and phosphorylation, but ribosylation was not observed.