Non-tuberculous mycobacteria (NTM) are regularly isolated from patients' sputum specimens submitted to diagnostic bacteriology laboratories. NTM have not been shown to be transmitted between persons, and thus the recognition of such infection does not require involvement by public health agencies responsible for control of infection and disease due to Mycobacterium tuberculosis. However, there is often a delay between the time of culture of mycobacteria and its precise identification as NTM. During this period, public health actions may be initiated and are later proven unnecessary after the subsequent identification of NTM. The objectives of this study were to determine the incidence of reports of non-tuberculous mycobacterial isolations in Jefferson County, Alabama, to examine some factors which led to the initiation of tuberculosis contact investigations and to estimate the cost of performing tuberculosis contact investigations. A review of Jefferson County Department of Health (JCDH) records and those of area hospital and private laboratories showed that although the incidence of tuberculosis in Jefferson County declined from 24.6/100,000 population to 18.1/100,000 from 1975 to 1981, the incidence of reported NTM isolations increased from 5.7/100,000 to 15.6/100,000. From 1978 to 1981, 94 patients with NTM infection and 69 of their contacts were prescribed therapy or preventive therapy for supposed tuberculosis. The estimated direct cost of these evaluations and short-term treatment was $18,721. From 1978-1981, 66 contact investigations involving 345 individuals were performed at an estimated cost to the JCDH of $21,166. Thus for this four-year period the estimated total cost of NTM-related activity was $39,887. The use of information relative to the cultural characteristics of the mycobacteria isolated could have resulted in avoidance of 32/66 (48.5%) of the contact investigations and $10,266 in costs. Areas of the United States with higher rates of tuberculosis as well as significant numbers of NTM-infected persons should be aware of ways to prevent unnecessary contact investigation. These measures include close communication with local laboratory microbiologists and encouraging the use of more rapid methods of mycobacterial recovery and identification. © 1987 The Society of Community Medicine.