authors' reply The term “opportunist mycobacteria” was suggested by Marks.1-1 He argued that M tuberculosis, M bovis, and M leprae were obligate pathogens which, if they did get into the environment, could not survive for any significant length of time. The other mycobacteria that cause disease in humans are, as Drs Davies and Grange say, free living environmental organisms and we are all continually exposed to them. However, comparatively few people become infected. Those who do usually have some pre-existing condition which predisposes them to infection—for example, chronic bronchitis and emphysema, bronchiectasis, previous tuberculosis, or some form of immunosuppression. The mycobacteria that are free living in the environment thus need an opportunity to cause disease—hence “opportunist mycobacteria”. It should also be pointed out that not all environmental bacteria cause disease. The nomenclature was discussed both by the Working Party and the full Joint Tuberculosis Committee; in both it was agreed by substantial majorities. This decision is also supported by the current and former directors of the Mycobacterium Reference Unit for England and Wales. We had hoped that the nomenclature argument about this group of mycobacteria could have been laid to rest once and for all after these decisions.