ee Th ee Tntroduction The potentially important sources of radioiodine (I131 ) in milk are: (a) atmospheric nuclear tests, (b) nuclear cratering experiments, (c) unscheduled ventings from underground nuclear tests, (d) releases from fuel reprocessing plants and other installetions. In 1963, Mechta (1) attributed the origin of zist in milk samples of the U. S. Public Health Service Pasteurized Milk Network (PMY) having 2300 pCi/l (picocuries of ist per liter of whole milk) to particuler sources during the period September 1961 through 1962. The present report updates the original report for the following six-year period. Waile radioiodine in milk has been measured because of its health implications, the primery purpose of this study is to account for the origin or yist in PMN milk samles. For present purposes, the absolute values of | the concentration are secondary to the assurance that the milk truly contains qi31, At least one PMN sample per week was routinely scheduled for analysis in each of about 60 milksheds; the sampling frequency increased to twice weekly when elevated concentrations were expected or found. each week may not reflect the highest level in milk. A single sample . The generally accepted limit of detection for iodine-131 analysis is 10 pCi/l. ‘This level is based on the random deviation in the analysis and count statistics and is defined as that: "level of activity which results in a 100% error at the 95% confidence level." However, if there are small quantities of unidentified isotopes present, such as naturally occurring radium-226, or unusual background fluctuations wnich are not accounted for in the analysis, a bias can be introduced in the results which will raise the minimm sensitivity. On this basis, a cut-off value of greater than 30 pCi/l was chosen as that representing a high level of confidence that iodine-131 was actually present (2). Many states operate their own milk networks; their results will be examined Tor support of the PMN findings. Because of earlier controversy, special attention is devoted to the possible sources of radioiodine from various nuclear activities at the Nevade Test Site (NTS). A local network operatec by the Public Health Service surrounding the test site provided data which bear on the interpretation of the PMY findings in terms ofa. Nevada source.