1. OBJECTIVE AND BACKGROUND For Operation Ivy, the Marine Survey Unit, Project 11.5 of Task Group 132.1, had as a major objective the evaluation of the contamination of living organisms following the Mike shot. An investigation of the residual radiation contamination of the fauna and flora of Eniwetok Atoll from the previous weapons tests was also undertaken. As Eniwetok Atoll has been used for several tests of atomic weapons since the last resurvey by the Applied Fisheries Laboratory group in 1949, it was essential to establish the level and kinds of residual radiation from the previous tests pefore the detonation and resultant contamination of the Mike shot. The data on the pre-shot contamination were obtained from samples of flora and fauna of the atoll and lagoon. In addition to the data accumulated on the level of radioactive contamination, it was necessary to determine the condition of the animal and plant populations in the areas chosen for sampling stations. The extensive construction program on the atoll since 1949 changed not only the surfaces of the islands but also modified many of the marine areas. Following Mike shot the studies were directed to the evaluation of: 1. The determination of the extent of the area contaminated. 2. The nature and kinds of the radioactive materials found in the various areas. -2-