Bronchoscopy specimens from 3 patients investigated during 1 wk at a large teaching hospital were culture positive for the same serovar of Mycobacterium intracellulare. Review of laboratory records and patient data suggested that only the first patient examined was a genuine excretor of mycobacteria; the isolates from the other 2 patients were false positives caused by inadequate disinfection of part of the collection apparatus between use. The investigations confirm the suitability of glutaraldehyde as a rapid, highly effective antimycobacterial disinfectant and illustrate the value of serotyping when dealing with mycobacterial isolates of questionable significance.