Mycobacteria-induced in vitro events reflecting human tuberculosis can contribute to the evaluation of the pathogenesis of Mycobacterium tuberculosis (MTB). In this study, we propose such an in vitro method based on live mycobacteria-induced cytotoxicity to human cell lines. When human lung-derived normal fibroblast cell line MRC-5 was infected with various strains of mycobacteria (M. tuberculosis H(37)Rv and H(37) Ra, Mycobacterium avium 427S and 2151SmO, and Mycobacterium bovis BCG Pasteur and Tokyo), the fibroblasts were killed by mycobacteria according to the degree of virulence. Other human originated macrophage (U-937, THP-1), myeloid (HL-60), and epithelial carcinoma (A549) cell lines exhibited a similar cytotoxic response to virulent mycobacteria. MRC-5 was most susceptible to virulent mycobacteria among various human cell lines examined. The cytotoxicity was enhanced by the proinflammatory cytokines, interleukin-1 (IL-1) and tumor necrosis factor-a (TNF-alpha), which in the absence of mycobacteria stimulate the growth of normal human fibroblasts. This in vitro evaluation system was applied to clinical isolates of drug-sensitive MTB (DS-MTB), drug-resistant MTB (DR-MTB) including multidrug-resistant (MDR-MTB), and M. avium complex (MAC). MTB strains (n = 24) exhibited strong cytotoxic activity, but MAC strains (n = 5) had only weak activity. Furthermore, there was no significant difference in cytotoxicity between DS-MTB (n = 11) and DR-MTB (n = 13). Collectively, these results suggest that this new in vitro system is useful for evaluating the pathogenesis of mycobacteria and that there was no difference in the pathogenesis between drug-susceptible and drug-resistant clinical isolates.