have been established within the past two years. However, sufficient data have been collected to indicate world-wide distribution of artificially radioactive material and that a good qualitative relationship exists between the total fission products and the strontium-90 activity which is falling out. The data indicate that on the average about 1 millicurie of _strontium-90 per square mile has fallen out up to 1 September 1954 in the United States. About 0.25 millicurie per square mile has been estimated for England and varies for other countries over the world from this value to a low of 0.03 millicurie per square mile in Africa. Stations in the Philippine Islands and other stations in that area show higher values, probably due to easterly winds at high altitudes from the Pacific Proving Ground conibined with the unusually high rainfall in the South China Sea area. Such rainfall is known to scavenge radio- active particles quite effectively from the troposphere. There is a large world area in the Communist-dominated countries where no samples have been collected. However, on the basis of the data available, it can be assumed that an average of 0.25 millicurie of strontium-90 per square mile has fallen to date in Europe, 0.1 millicurie per square mile in the Southern Hemisphere and 1.0 millicurie per square mile in the United States. It should be noted at this point that the total activity from fall-out falls off quite slowly after about one year following a detonation. The decay factor of tie is no longer a use- ful approximation much after that time, since the decay rate is then controlled by only the few long-lived isotopes remaining. For a period of from one to six months after a detonation, the rare earth elements constitute about 30% of the total activity. Domination of the activity slowly chenges so that after about one year a few specific isotopes such as cesium-137, strontium-90 and a few of the longer lived rare earth elements determinethe effective over-all decay rate. The effect of the environment upon which the fall-out occurs is to tend to fractionate certain elements, particularly strontium-90, so 58