Biologic substances occurring in such minute quantities that they eluded previous methods can now be assayed in vitro by a variety of nuclear techniques. Dr. Murphy discusses the quantitative and biologic assays made possible by the use of proteins to“capture”the target substances, or“ligands.”Dr. Wagner reviews the uses of radioactive carbon in the evaluation of metabolic events and anomalies in vitro rather than in vivo, and also discusses other methods, employing radioactivity measurements, that appear to offer significant potential for clinical application.