1. Tissue mast cells were found in 7 of 2800 human bone marrows studied. 2. Tissue mast cells represented 1.0 per cent or more of the marrow cells in 3 of the cases, and each of the 3 exhibited some degree of marrow depression, as evidenced by anemia, leukopenia and/or thrombocytopenia. 3. The morphology and histochemistry of tissue mast cells in human bone marrow are described. 4. A relationship between tissue mast cells and marrow cellular depression is suggested based upon a possible inhibitory action of heparin upon local cell growth.