Lymphocytes obtained from rabbit lymph nodes sensitized to bovine gamma globulin produce in vitro the lymphokine macrophage fusion factor (MFF) which mediates the fusion of approximately 100% of normal alveolar and oil-induced peritoneal macrophages. Giant cells (GC) of Langhans and foreign body type form large syncytia containing as many as several hundred nuclei per cell. Nuclei of GC appear more spherical and larger than those of the normal mononucleated macrophages, and they possess several prominent nucleoli. Giant cells of peritoneal macrophage origin show enhanced intracytoplasmic vacuolization. Normal macrophages cultured as a monolayer in MFF-rich supernatants form cell clusters which progressively fuse during the 24-hour incubation period. A signoid dose-response curve was obtained for cell fusion with MFF-rich supernatants possessing high titers, ie, the latter supernatants undiluted partially inhibited macrophage fusion. MIF-like activity was detected in MFF-rich supernatants as well as a factor(s) which inhibited 3H-thymidine uptake by giant cells.