A multidrug-resistant strain of Mycobacterium tuberculosis (MDR-Tb) and Multidrug-sensitive Mycobacterium tuberculosis (MDS-Tb) isolated from humans were injected subcutaneously into the guinea pigs. One set of infected lungs of animals was fixed in 10% formalin pH7.2, while the other set was treated with 75% ethanol 2 hours prior to fixation in 10% formalin. After six months of fixation, MDS-Tb strains were completely inactivated by both methods of fixation. MDR-Tb strains resisted inactivation by 10% formalin but were inactivated by the treatment with 75% ethanol 2 hours prior to 10% formalin fixation. MDR-Tb was still viable after six months in tissue preserved in 10% formalin only. Pretreatment of tissue with 75% ethanol prior to preservation in 10% formalin gave a better safety precaution against MDR-Tb in tissue. Of the 50 infected patients studied during the period, 4 (8%) harbored MDR-Tb, while 46 (92%) carried MDS-Tb.