Health and disease in living systems depend on the dynamic interplay of thousands of biochemical substances occurring in living systems in concentrations ranging from parts per hundred to parts per billion or trillion. Radioimmunoassay (RIA) is a highly specific and sensitive technique for measuring the concentration of such biochemical substances. It represents one of the most dramatically expanding areas of medical diagnosis and research. Reviewed here is the recent progress on RIA, with emphasis on methodology and on its adaptation and application in developing countries. The number of biological substances (ligands) being assayed by RIA continues to expand. RIA is central to diagnosis, epidemiology and research. It has been successfully applied in the study of parasitic and infectious diseases. Introduction of RIA into developing countries, for which the Agency's help is sought and given, presents numerous problems: personnel, equipment, adaptability of techniques to local needs, and public support.