A steady rise in the number of isolations of Mycobacterium xenopi from patients in a general hospital led to an examination of water taps. Most patients had been accommodated in a group of wards which had a common water supply. This organism was recovered from 35 of 69 outlets, mostly from hot and mixer taps in those wards. Mycobacterium kansasii was also isolated from 14, mostly cold and mixer taps. Ten strains of Myco. xenopi were recovered from 131 taps sampled at 10 other locations. We conclude that colonization of water supplies by mycobacteria is a likely source of contamination of clinical specimens.