Following an intra-atrial injection, microspheres of 15μ diameter could be recovered in large numbers from the isolated oropharynx, trachea, or bronchi. By stripping tracheal or bronchial blood supply or by diverting aortic output, their appearance was suppressed. Following aortic blood diversion, few were found in isolated bronchial washings. These had originated at the alveolar level via the pulmonary artery. Following intra-aortic injection, microspheres were readily recovered from the stomach and urine. When either the terminal aorta or renal arteries alone were ligated, microspheres appeared in the urine. When both were ligated none appeared, indicating they passed via both kidneys and cystic arteries. Microspheres bear physical similarities to tumor cells and other formed blood elements. If tumor cells migrate similarly, this mechanism could partially explain their rapid depletion from the circulation and also indicate a new mode of metastatic spread.