Free cancer cells can be identified in the circulating blood by removing the erythrocytes with a heparin-fibrinogen mixture, centrifuging the plasma, and examining smears made from the buffy coat. In a series of 179 patients with advanced cancer, one-half of the samples of peripheral blood studied in this way yielded recognizable tumor cells. These metastasizing cells with diameters of 10 to 30 μ could deform and pass through apertures less than 10 μ in diameter and hence could traverse capillary beds. Washings of the peritoneal cavity before and at the end of celiotomy in patients with malignant intra-abdominal lesions similarly yielded tumor cells. These findings prove the need of care to prevent the iatrogenic spread of malignant cells. They also encourage the continued search for cyfotoxic, anticancer agents that can be administered to patients with malignant disease to prevent metastasis.