INTEREST has increased over the past few years in the use of technetium 99m as a radioactive tracer material for organ and tumor localization studies (1). Initially, attention was focused on this nuclide because of its favorable physical properties for biological applications (2). Subsequent chemical and physiological studies have revealed remarkably versatile characteristics which may make Tc99m the radioisotope of choice in a variety of clinical applications (3–13). This report summarizes the progress made in our laboratory during the past four years in investigating this isotope. Chemically technetium belongs in group VII-A along with manganese and rhenium, the resemblance to the latter being particularly close. The physiologic behavior of technetium compounds has not been extensively studied except for the pertechnetate. This rather stable ion resembles iodide very closely in its initial distribution in the body (14). Like iodide it is selectively concentrated in the thyroid, salivary glands, and stom...