A technic has been described in animal experiments whereby the circulation to an entire lung or lobe has been compromised either by clamping the vessel or by inducing an embolus. The intravenous injection of radiating micro-spheres results in their uniform distribution throughout all the normal lung tissue, but not to the lobe or lung in which the artery has been occluded. Scintiscanning permits the localization of the occluded artery and offers a simple method of diagnosing pulmonary embolus. Studies are in progress to ensure the safety of the procedure in man.