A highly functional, transplantable neoplasm of mast cells is described. It causes solitary slowly growing tumors localized at the site of graft in muscles and subcutaneous tissues.

Mast cell leukemia results when isolated cells are injected intravenously. The blood of mice with mast cell leukemia produces solid mastocytomas when injected intramuscularly.

These and other observations suggest the essential identity of blood and tissue mast cells and suggest that mast cells are an independent cell type with primary residence in tissues outside the hemopoietic organs.

In mastocytomas, heparin, histamine and serotonin are present in great quantities. Some histamine is released and partly excreted in the urine.

In the mast cell leukemias, histamine plasma levels are slightly raised and urine levels are highly elevated.

Plasma heparin values may be slightly raised in mice with mast cell tumors and are greatly increased in mast cell leukemias without a hemorrhagic state.

The liver histamine and heparin values appear to be related to the number of infiltrating mast cells.

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