Setting: The success of Mycobacterium tuberculosis as a human pathogen depends on its ability to tolerate and perhaps manipulate host defense mechanisms. Objective: To determine the induction of tumour necrosis factor-alpha (TNF⍺), a central mediator of immunity, by human monocytes infected with virulent M. tuberculosis, M. leprae and attenuated M. bovis BCG. Design: Mycobacteria-induced cellular activation pathways of TNF⍺ production was investigated using an inhibitor of protein tyrosine kinase (PTKs) and an inhibitor of mitogen-activated protein (MAP) kinases. Results:TNF⍺ production was significantly lower during infection with virulent M. tuberculosis than with BCG and this differential response was independent of mycobacterial viability. TNF⍺ production involved the PTK and MAP kinase pathways. Reduced TNF⍺ induction by M. tuberculosis was associated with a reduction in the extent and duration of phosphorylation of extracellular-signal regulated kinases (ERK 1/2). Infection with M. leprae triggered low and transient ERK 1/2 activation as well as low TNF⍺ production. Conclusion: Maintenance of the differential response in both live and heat-killed preparations suggests that the reduced TNF⍺ response associated with virulent mycobacteria is due to differences in the presence of components capable of triggering host pattern recognition receptors, rather than events associated with phagosome trafficking or the active release of intracellular modulators.