Although reports of cancer cells in the blood stream have appeared sporadically in the medical literature over the past 100 years, only recently has it been possible with newer techniques to isolate and definitely identify these cells. This study is a continuation of work initially reported by Cole, Packard, and Southwick,1 in 1954. At that time cancer cells were isolated from fluid perfused through the major artery and collected from the major vein of colonic specimens containing carcinoma (Fig. 1). As a result of this work the study was amplified to include the examination of whole blood samples collected during the observation and treatment of patients with malignant disease. Such an approach has made it possible to study certain prognostic and therapeutic features, including response to surgery and chemotherapy. Methods Collection of Blood Samples. —Peripheral blood samples are drawn from the antecubital vein. Blood samples from the veins draining