terms of the conditions likely to be encountered in the field as was the case for the accidental human exposures at CASTLE. If in the case of the first CASTLE shot, the instrument readings in milliroentgens per hour at 7 to 9 days post-shot are extrapo- lated back to shot day and the value thus obtained integrated over the period of exposures using the + f decay law, the values obtained compare favorably with dosages that would be estimated on the basis of the biological responses observed. The problem of measurement is not © so simple and it seems possible that there may be factors omitted from the oversimplified estimation procedure just cited that would tend to lower the values obtained. Among these are: (1) The travel time of fall-out, which may vary from estimates made. (2) The gradual build-up of fall-out, which makes determination. of a single “arrival time" uncertain. (3) The shielding effect of special environmental conditions, such as buildings, trees, and clothing. These might be counteracted by other factors not directly considered which would tend to raise such a value: (1) The geometry of exposure. (2) The adhering of particles to clothing and skin. (3) Increased intensities below the 3 foot measuring level. (4) The biological recovery factor in prolonged exposure times. Thus a cancellation of errors may permit by a fortunate coincidence the use of a simple straightforward calculation to obtain a quite valid estimate of the dose involved. The temporary epilation in the Marshallese suggests the dose to exposed skin on Rongelap was approximately 10,000 rep of beta radiation. Because of evidence of completely intact skin under clothing, the dose to the protected skin was probably less than 5000 rep. Thus the CASTLE data suggest that the clothing worn by exposed persons pro- _ 83