OBJECTIVES : Can the characteristics of persisters in cultures of Mycobacterium tuberculosis also be found in bacilli from the sputum of pulmonary tuberculosis patients? The objective of this study was to explore whether the ability of persisters to grow in liquid but not on solid culture media, as in 100 day static cultures, can also be found in bacilli in sputum. METHODS : Serial dilutions of homogenized sputum obtained from patients before or during the first week of treatment were inoculated into broths to estimate the probable number of organisms and onto plates to give colony counts. RESULTS : Cultures in broths grew slowly to reach a maximal count at 12 weeks of probable numbers about 10-fold higher than the colony counts on plates, which did not grow beyond the initial count at 3-4 weeks. No such excess growth in liquid medium was found with control log-phase cultures. CONCLUSIONS : About 90% of the bacilli in sputum are persisters that can grow in liquid media but not on solid plates.