A transmissible neoplasm of mice characterized by malignant cells resembling histiocytes (monocytes) is described. The morphology of these cells and the microscopic appearance of the lesions are similar to those of human neoplasms formed by histiocytes.

The malignant histiocytes form tumor-like masses in the liver and spleen and infiltrate these and other tissues. They are present in small numbers in the blood of many mice when the disease is far advanced. The malignant cells have scant phagocytic ability. The fixed cells of the host (endothelial cells and fibroblasts) have no significant part in the production of the lesions of the disease.

Transmission is readily accomplished when material containing the malignant histiocytes is used for inoculations, but fails in their absence. Attempts to demonstrate a cell-free transmitting agent have been unsuccessful.

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