Tuberculosis (TB) is a prominent opportunistic infection in HIV-1-infected subjects and enhances HIV-1 replication. TB is associated with excess monocyte chemoattractant protein (MCP)-1 and tumour necrosis factor (TNF)-α activity in situ, both of which are implicated in transcriptional activation of HIV-1. The role of MCP-1 and TNF-α in activation of HIV-1 during TB and by Mycobacterium tuberculosis (MTB) in mononuclear cells from HIV-1/TB subjects with pleural TB was examined here. Extremely high levels of MCP-1 (as compared with TNF-α) protein and mRNA were found in pleural fluid and pleural fluid mononuclear cells. Levels of MCP-1 mRNA were sustained during in vitro culture of pleural fluid mononuclear cells. Neutralization of MCP-1 (but not TNF-α), resulted in inhibition of MTB induced HIV-1 gag/pol mRNA. Neutralization of both MCP-1 and TNF-α, however, abrogated the effect of anti-MCP-1 antibody on HIV-1 mRNA. LMP-420, a small molecular transcriptional inhibitor of both TNF-α and MCP-1 expression, did not reduce MTB-induced HIV-1 expression. These data imply that MCP-1 activity may be critical to activation of HIV-1 at sites of TB. An interplay of MCP-1 and TNF-α is also suggested.