Radioactive tracers have had a profound impact on scientific research during the past twenty years, particularly in bio chemistry and allied fields. The rapid evolution of sophisticated instrumentation and labeling techniques during the past ten years has made possible on a routine basis exquisitly sensitive experiments which with other techniques would require her culean efforts on the part of the investigator. The Symposia on Advances in Tracer Methodology, which have been held regularly since 1957, provide for research workers in biochemistry, pharmacology, endocrinology, and medicine a focus on the numerous developments in the tracer field that are usually scattered throughout the scientific literature. Volume 3 is arecord of the proceedings of the Ninth and Tenth Symposia, which were sponsored by the New England Nuclear Corporation and the Packard Instrument Company, Inc. Recent advances in tracer methodology, as reflected by the papers in this volume, have been mainly concerned with im proved analytical techniques, such as liquid scintillation count ing, autoradiography, and chromatography, and with novel ap plications of labeled compounds in biochemical and clinical problems.