It is generally accepted that cells from malignant growths are at some stage in the progress of the growth carried by the blood stream from the primary tumor to other parts of the body. Some lodge and form metastases; presumably the larger number are destroyed by lytic properties of the blood stream or tissues and many potential metastases are thus prevented. Possibly large numbers of such cells are present in the blood stream. On this assumption we have studied the blood in cancer cases. The picture of a patient with carcinoma of many months duration, riddled with metastases, is too well known to need detailed description. In such cases cells have been seen in fixed tissues, filling the lumen of vessels, and in many instances invading the vessel walls. Schmidt in 1903 made microscopic examinations of the lungs in 41 cases of advanced malignant disease, and in 15 found tumor emboli in the pulmonary arteries. Pathologists use the evidence of blood vessel invasion as one of the criteria in establishing the diagnosis of carcinoma of the thyroid gland. In a case of carcinoma of the stomach studied by Quensel a small tumor embolus the size of a pea was found in the right auricle of the heart.