Because of the difficulty in the past of culturing blood for mycobacteria, blood cultures have not been used for either diagnostic purposes or monitoring response to therapy in patients with mycobacterial infections. It has recently been demonstrated that blood culture is useful for detecting sepsis with Mycobacterium avium-intracellulare in immunosuppressed patients. This study employed techniques similar to those used for the detection of mycobacterial sepsis in immunosuppressed patients (use of Isolator concentrate and Bactec medium with radiometric detection of growth) to see if blood culture would be a useful technique either for diagnosis or for following the course of patients with Mycobacterium tuberculosis infections. In 12 patients with M. tuberculosis infections, no positive blood cultures were obtained. Therefore, routine blood culturing of patients with M. tuberculosis infections does not appear to be a useful procedure.