The 15th Latin American Symposium ''laS held in Brasilia (FD) on J1UY 18-22, 1977, on a topic of great interest for agriculture, especially in the tropics. Many new developments have taken place in the field of research in N2 fixation during the last few years. They "Tere made possible by the improved methods of measuring of nitrogenase activity, progress in genetic engineering fields and the increased interest in taking advantage of natural sources for biological nitrogen fixation. The approach used in this Symposium together with the one held four months earlier in Brookhaven on ;'Genetic Engineering for Nitrogen Fixation" gives an interesting picture of the present status of nitrogen fixation from two diverse approaches. This is my 20th year visiting Latin i'-J11.erica. I am most impressed with the tremendous development which has taken place during these years in Latin American science. I want to congrat­ ulate our Brazilian colleagues for arranging this excellent and timely symposium and its excellent organization. These symposia are a cooperative effort between our Latin colleagues and scientists allover the world. They are made possible by excellent local support and support by a number of international agencies and several groups in the United States. Earlier symposia in this series are listed on pp. viii-x. Alexander Hollaender vii viii PREFACE I. International Symposium on Tissue Transplantation--Santiago, Vifia del Mar, and Valparaiso, Chile. Published in 1962 by the University of Chile Press, Santiago; edited by A. P. Cristoffanini and Gustavo Hoecker; 269 pp.