THE RATE OF ACCUMULATION of a substance in any region of the body is directly proportional to the blood flow to the region, provided the substance is completely removed from the blood and is not metabolized during the period of observation. In the present study, we have successfully used this principle to detect alterations in blood supply to various regions of the lung as a result of certain diseases. The studies were made possible by the development of a new radiopharmaceutical, labeled macro-aggregated albumin (MAA), which has been found to be both safe and effective. Materials and Methods In the initial studies radiodine (I131) was employed as a label for the macro-aggregated albumin. Subsequently we have used chromiumlabeled albumin, because radioactive chromium (Cr51) emits a mono-energetic gamma ray (320 kilo electron volts) that is preferable for scintillation scanning. In addition, the absence of beta emission decreases the radiation