INTRODUCTION The program of the Atomic Energy Commission on environmental contamination from weapons tests is designed for the over-all evaluation of the hazard to humans from test operations. It is limited to studies of the deposition of activity at long range rather than the problems associated with immediate, close-in fallout. The program has largely been a study of Sr*, Since considerations based on experience and measurementindicatethat it is the isotope of greatest potential hazard. The data on fallout were last summarized in the report, The Nature of Radioactive Fallout and Its Effects on Man (Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, May 27-—June 7, 1957). The next important summary will be in the report to the United Nations General Assembly from its Special Scientific Committee on the Effects of Atomic Radiation, which should appear during the current year. The present report has been prepared by the Health and Safety Laboratory, under the direction of the Division of Biology and Medicine of the AEC to summarize, in tabular form, the data available on the monitoring of Sr®’ and Cs!" levels in man and his environment. Many of the studies reported are documentaryin nature, i.e., they are designed to produce a permanent record of the concentration of Sr*® existing in various materials at the time. Naturally, other ideas in addition to specific monitoring programs are pursued where they may be fruitful as an aid in understanding the processes involved. However, the material presented here is largely the result of surveys rather than planned experimentation. The data reported is not an evaluation of the hazard from weaponstesting. The final interpretation of data is a medical and biological problem, requiring studies of the uptake of Sr® by man from his environment and a knowledge of the level of Sr*° that may be considered permissible in man. From the data presented, however, it is possible to obtain an understanding of some of the steps in the process leading to possible damage. Even in the limited field of monitoring, there are many scientific problems that arise in sampling, radiochemical analysis, and data evaluation. These problems are quite apart from the more controversial interpretation of the possible hazard to man: (1) It is first necessary to know, to the required degree of certainty, what the actual levels of Sr® contamination are in various parts of the environment. The sampling should be directed not only toward obtaining an estimate of the average contamination but also toward the probability that much higher than the average values may exist in a small portion of the environment. Fallout is not uniform and possible hazard to relatively small groups of people must be considered; facilities for extensive work of this kind have not been available. (2) The analytical process is extremely involved, requiring the utmost in care and the highest quality in measuring equipment. The radiochemical properties of Sr* and its extremely small concentrations in samples make the analysis a slow process, and, under the best conditions, there is a considerable time lag between sampling and final reporting of results. This is further accentuated by the need for accuracy, which means that a system of checking and cross-checking of all data is a primary requirement. (3) The evaluation mentioned here is merely the consideration of the validity of the analytical data rather than its final interpretation. Such evaluation requires not only a knowledge of the quality of the radiochemical analysis but also a knowledge of how the data received fit into the known pictures of meteorology, soil chemistry, plant uptake, and thelike.