This publication contains papers presented at an international symposium held in Vienna in 1991. The meeting reviewed the use of isotopic methods in animal nutrition for developing optimal diets and feeding strategies. Considerable emphasis was also given to the role of hormone measurement by RIA for improving reproductive efficiency, and the role of enzyme immunoassay, ELISA, methods for diagnosing animal diseases and monitoring disease control programmes. Several reviews deal with isotopic methods in molecular biology, particularly their role in diagnosis and development of new vaccines.