The inactivation of smears that contain Mycobacterium tuberculosis for microscopy before removal of the material from a biosafety cabinet is an important safety factor in preventing the potential transmission of tuberculosis to laboratory workers. The fixing and inactivating properties of heat flaming, 70% ethanol, and 1, 3, and 5% phenol in ethanol for smears containing M. tuberculosis were investigated. Heat flaming failed to inactivate the smear material, whereas 5% phenol in ethanol successfully fixed and inactivated all smears containing M. tuberculosis both from concentrated sputum samples and from culture material.The potential hazard of laboratory work with Mycobacterium tuberculosis is well recognized. There are numerous records of laboratory-acquired tuberculosis (TB) infections, with aerosols and skin punctures being the most common reported routes of transmission (10,13,15,18,19,24). The resurgence of TB in industrialized countries in the past decade has resulted in an increasing number of specimens and cultures being tested in laboratories, thereby enhancing the potential of accidental exposure for laboratory staff.Stringent safety precautions must be followed for laboratory work with M. tuberculosis (6,14). There remains one part of the routine laboratory process, however, where the hazard for potential exposure has not been addressed-unstained smears. Specimen and culture manipulations for mycobacteriological analyses, including the preparation of smears, are carried out within a biosafety cabinet (BSC). The smears are usually then heat fixed by passage of the slides through the flame of a gas burner or by placement on a hot plate. After the smears are fixed, they are removed from the BSC to a staining sink or they may be stored, transported to a reference laboratory, or used in proficiency testing panels.Several studies have shown that heat-fixed smear material, whether fixed by flaming or for 2 h at 65°C on a hot surface, still contains viable bacilli (1, 4; L. R. B. Giacomelli, S. R. Sespede, A. M. W. Barreto, and C. L. Cardoso, Abstr. Clin. Microbiol., abstr. C-129, p. 131, 1999). Our data from this study confirm that M. tuberculosis is still viable after flame fixing of smear material. Additional problems with heat fixing are that the use of a flame inside a BSC is not recommended and that 2 h of fixing on a hot plate is cumbersome and time-consuming when large numbers of slides must be fixed.When slides containing viable M. tuberculosis smear material are removed from a BSC, laboratory staff can be exposed to infective material if slides are broken, thereby possibly generating aerosols or skin penetration, or the smear material may flake off slides, thus potentially infecting the worker and contaminating the laboratory environment. Staining methods used for acid-fast bacilli (AFB), whether fluorochrome or carbol fuchsin based, are known to kill mycobacteria. However, staining is done over a sink outside the BSC, necessitating the prior removal of unstained smears from the cabinet.The purpose...