1. Foreign body giant cells may be produced in vitro by the addition of foreign objects such as lycopodium spores and cotton fibers to cultures of chick embryo spleen. 2. These giant cells are formed by the fusion of large mononuclear wandering cells, probably endothelial cells and pulp cells. Connective tissue cells do not take part in their formation. 3. The large giant cells sometimes seen spread out over the cover-glass in cultures of chick embryo spleen are probably foreign body giant cells, the cover-glass acting as the foreign body.