Mycobacterium tuberculosis alone induces small, donor-variable amounts of tumor necrosis factor alpha (TNF-) from primary human monocytes in vitro. However, TNF- release is increased 5- to 500-fold when fixed activated T cells (FAT) or their isolated, unfixed membranes are added to this system. This FAT-induced synergy was at least as potent as that induced by gamma interferon (IFN-) at 100 U/ml. FAT-enhanced TNF- production is at least in part transcriptionally mediated, as reflected by quantitative changes in TNF- mRNA between 2 and 6 h poststimulation. Unlike IFN--cocultured cells, FAT-treated monocytes appeared not to have enhanced TNF- message stability, suggesting that de novo transcription may be involved in this effect. Furthermore, M. tuberculosis alone induced only minimal DNA binding of monocyte NF-B, but cells treated with M. tuberculosis and FAT potentiated NF-B activity more effectively. It is therefore possible that one mechanism by which FAT synergize with M. tuberculosis to stimulate TNF- production is via NF-B-enhanced transcription. These data strongly suggest that in the interaction of cells involved in the immune response to M. tuberculosis, T-cell stimulation of monocyte TNF- production involves a surface membrane interaction(s) as well as soluble mediators.