Supernatant fluids from cultures of BCG-sensitized rabbit lymph node and spleen cells contained a factor that strongly agglutinated normal rabbit alveolar macrophages within 3 min at room temperature. In contrast, fluids from nonsensitized cell cultures did not agglutinate normal rabbit alveolar macrophages. This factor was designated macrophage-agglutinating factor (MAgF) because it is similar to the previously described factor found in lung lavages of rabbits exhibiting a BCG-induced pulmonary delayed hypersensitivity reaction. The kinetics of MAgF production in vitro by sensitized lymph node cells and its inhibition by puromycin and actinomycin D suggest active synthesis; sensitized spleen cells exhibited kinetics resembling release rather than synthesis. Studies on purified lymphocyte and macrophage populations from sensitized spleen and lymph nodes indicated that lymphocytes are responsible for MAgF production. However, MAgF production was not induced in normal cells incubated in vitro with concanavalin A or phytohemagglutinin. Fractionation of cell culture supernatant fluids in Sephadex G-100 or Ultrogel AcA-34 clearly separated MAgF from migration inhibition factor; MAgF was present in the void volume of the eluates, suggesting a molecular weight of over 400,000, whereas migration inhibition factor was recovered in the same peak as albumin. The role of MAgF in vivo is unknown, but it is postulated that it may cause the adherence of macrophages during granuloma formation.