made by people who had intimate and current knowledge of the original measurements. Second, we will use these estimates to calculate population exposure by communities; we hope such data may be useful to epidemiologists. A third purpose is to identify those locations that received the largest population exposures and those weapons tests that produced the largest population exposures. A subsequent paper will address the population exposures that have resulted from NTS-related activities after the Hardtack II test series ended in 1958. Shieien (Sh181) recently published his estimates of population exposure for activities at the NTS between 1951 and 1970. His results are based upon a different calculational model and he did not include several exposed communities that were included in the TMCEFD tabulation. METHODS About half of the population exposure during the 1951 to 1958 period was due to the UPSHOT-KNOTHOLE series in 1953. It is important to note that during this series only very few measurements of exposure were made by the use of film badges or other integrating devices. Rather, measurements of open-field external exposure rate were made and a calculational model was necessary to convert to estimates of human exposure. External exposure rates were typically measured with the AN/POR-T1B ionization-chamber instrument when the rates were >10 mR/hr or the MX-5 Geiger-Mueller tube instrument when rates were <10 mR/hr. Because readings were made at many times post detonation when the external exposure rate was changing rapidly with time, it was desirable to normalize to a common time in order to construct isopleths. 204 The