INTRODUCTION In April 1975, Brookhaven National Laboratory initiated an external survey of Bikini Atoll in order to obtain information concerning the ambient external radiation levels resulting from the mid 1950's weapons testing program and to make dose equivalent commitment determinations for the individuals living in the surveyed area. From 1975 to 1977, measurements were made to provide sufficient information on the external exposure received by the Marshallese people. Most of the information concerning Bikini and Eneu Islands was obtained in April 1975, when environmental ionization chamber measurements were made. In addition, thermoluminescent dosimeters (TLDs) were placed in the field and exposed for six months at Bikini Island to verify the uniformity of the exposure. Other groups assisted in these surveys. The team from Lawrence Livermore Labora~ tory (UCRL) made a detailed survey of Bikini and Eneu Islands in June 19754,9, and they refer to the information presented in this report as BNL unpublished data. In general, their results are substantiated by the exposure and dose equivalent commitments calculated here. The equipment used in 1975 consisted of a Reuter Stokes environmental radiation monitor model RSS-111 and a Baird~-Atomic scintillation detector con- sisting of a sodium iodide detector (2.5 cm in diameter by 3.9 cm in length) connected to a ratemeter readout. Portable survey meters were used to help locate gross changes in the external exposure rate. Lithium fluoride thermoluminescent dosimeters were left on Bikini Island and retrieved in December 1975. Environmental exposure levels were assessed via the RSS-111 and a Nal gamma spectrometer whose purpose was to determine the photon energy distribution and to compensate for the nonlinearity in the RSS-L1l instrument response. This report presents all of the external exposure data collected to date for Bikini Atoll by BNL. These data have been used to make external exposure es- timates for the people living on Bikini Island, and the BNL data have been com~ pared with UCRL data? for Bikini Atoll. INSTRUMENTATION AND METHODS A) Ton Chamber Measurements All environmental exposure rate measurements were obtained with a Reuter Stokes environmental radiation monitor model RSS~111, which is designed to measure environmental radiation as low as 100 UR/yr. The RSS-111 consists of a spherical high pressure ion chamber filled with argon to a pressure of 25 atm. Incident radiation produces ion pairs within the active volume of the chamber which result in an ionization current. The current flow is measured by an electrometer and is directly related to the free air ionization rate®. *1%. The active volume of the stainless steel lonization chamber is known to The ionization current produced in the chamber is a function of incident radiation from an external field, cosmic-ray response, and contamination present