Cell-free supernatant fluids obtained from BCG-sensitized lymph node cells (6 x 10(6) cell/ml) incubated in tissue culture flasks containing heat-killed BCG (5 mug/ml) induced extensive development of multinucleated giant cells when incubated with normal alveolar macrophages. In contrast, supernatant fluids obtained after incubation of similar samples of the same lymph node cell population in flasks with (i) no mycobacteria, (ii) heat-killed Escherichia coli, or (iii) heat-killed Bacillus subtilis failed to produce giant cells when added to normal alveolar macrophages. Giant-cell formation was observed in experiments using Nocardia brasiliensis as the eliciting antigen, indicating cross-reactivity between the antigens of this organism and BCG. These experiments indicate that BCG-sensitized lymphoid cells produce a soluble macrophage fusion factor after specific antigen stimulation. The fusion factor is a nondialyzable substance which is resistant to heating at 80 C for 30 min.