1. A simple technique is described for isolation and estimation of number of erythroblastic islands (EBI) in rat bone marrow. The number of EBI decreases during hypertransfusion polycythemia, and increases during experimentally-induced hemolytic anemia. It has also been possible to maintain EBI in vitro, under culture conditions, for up to 36 hours. 2. Each EBI contains a central cell having the character of a macrophage-histiocyte which adheres to glass and spreads, while still retaining its connections with the erythroblasts. 3. In an EBI the erythroblasts are arranged in the form of concentric rings, with the outer cells being more mature than the inner. The outermost layer is frequently made up of reticulocytes even in the isolated island. The erythroblasts of a single ring appear to be at the same stage of maturation which suggests that they are derived from the same stem cell. 4. Using a coordinate reference system, studies of nuclear expulsion were made, by both optical microscopy and scanning electron microscopy.