FOREWORD In November 1961 the Division of Biology and Medicine held its first formal conference to review and discuss its research program in atmospheric radioactivity and fallout from nuclear weapons tests. Since new work had been accomplished and several more series of nuclear tests had been performed, a Similar conference was held in the fall of 1964. The confer- ence and this report of its proceedings are intended to highlight the recent major research findings in this area of the Division’s program. It is hoped that the conference has assisted in the evaluation of fallout problems and that these proceedings will provide a useful reference to those having an interest in them. , I wish to express my appreciation to all who attended the conference and for their participation in the presentation of papers and subsequent discussions. Charles L. Dunham, M.D. Director, Division of Biology and Medicine U.S. Atomic Energy Commission ill