1. The intravenous injection of I131-labeled heterologous anti-leukocyte and anti-bone marrow antibodies into rats resulted in a high specific in vivo localization in the bone marrow, indicating the presence of localizing anti-body.
2. No in vivo localization in lungs was demonstrated, in possible contrast to the earlier concept of pulmonary emboli of clumped leukocytes sensitized with antibodies in experimental immunoleukopenia.
3. Heterologous anti-leukocyte and anti-bone marrow antibodies injected intravenously disappeared from plasma and fixed to peripheral leukocytes and bone marrow cells within 1 hour.