= UNCLASSIFIED ABSTRACT The physical and chemical properties of the particulate matter deposited following Mike shot, Operation Ivy, together with its distribution in time and area, were investigated. Total fall-out and differential fall-out collectors were installed on islands, anchored lagoon floats, and free-floating sea stations about the detonation point. All collected samples were analyzed at the U. S. Naval Radiological Defense Laboratory. All the samples of primary fall-out collected were in a cross-wind direction from the detonation point; secondary fall-out samples were collected to distances of 600 miles from this point. Radiation levels as high as 800 r/hr at 2 hr were found 3 miles from the detonation point. No positive evidence of the occurrence of a base surge was found. The primary fall-out was a dry or semidry particulate of compoundsof calcium with entrapped fission products. This particulate underwent a chemical change when in a sea-water environment, causing it to become very firmly attached to any surface it touched. Particles were found ranging in diameter from less than 10 » to more than 5000 yu. There was no indi- cation of size fractionation of the particles with distance and only meager evidence of size fractionation with time. The quantity of primary fall-out varied from more than 20 g/sq ft at 4 miles cross wind to 0 g/sq ft at 15 miles from the shot point. The time of arrival along the cross-wind direction was completely independent of the distance from the shot point. The secondaryfall-out originating in the stratosphere was less than 25 » in diameter and arrived 2 to 5 days after shot time. None of the stations where secondaryfall-out was collected reported a gammadoserate greater than 10 mr/hr. UNCLASSIFIED