1. Pure strains of mononuclear leucocytes were isolated from the blood of adult chickens and keptin active condition for nearly 3 months.
2. The cultures were composed of large mononuclear leucocytes which migrated and proliferated in vitro at a slower rate than fibroblasts. The cells had no tendency to form a tissue, as do fibroblasts and epithelial cells. They were much less resistant than fibroblasts.
3. Differentiation of the large mononuclears into cells assuming the appearance of fibroblasts took place under certain conditions.
4. The activity of the large mononuclears was increased by embryonic tissue juice and inhibited by homologous serum.