analysis. The measurement of radionuclides in sea-water is difficult because of the necessity of separating and concentrating atom quantities of the nuclides from gram quantities of sca salts. z §-emitting radionuclides, such as phosphorus-32 and strontium-90, require complete separation of the nuclides from the salts to reduce the absorption of B-particles by the sample. Photons from y-emitting radionuclides. such as chromium-51, zinc-65 and zirconium-95--niobium-95, are not absorbed in the sample; but the amount of salt in a sample large enough to contain a measurable quantity of a radionuclide usually is 60 great that the sample geometry is reduced to an unacceptable value. Therefore, radionuclides for yspectrum analysis also are separated from thesalt. Our purpese here is to discuss methods used at this Laboratory for the analyses of radionuclides in sea-water and to present the results of these analyses. The methods and results of analyses of Christmas Island samples have been reported by Welander and Palumbo!, and those of the Columbia River sainples are reported by Seymourand Lewis?; however. they arc reviewed here to present a sumunary of the Laboratory’s methods and results. In addition, values for strontium-90, as determined by other investigators, have been included to indicate the world-wide distribution of this radionuclide in the surface waters of the oceans. Collection and preparation procedures. Surface seawater samples were collected from three areas in the Pacific Ocean: in the vicinity of Christmas Island (approximately 2° 8.-2° N. latitude and 153° W.-168° W. longituds); within 20 amiles of the Columbia River (approximately 46° N. and 124° W.); and 250 miles off the Oregon Coast (approximately 45° N. and 130° W.). The Christmas Island samples were collected during the summer of 1962 and the samples taken near the Columbia River were collected during January 1961June 1962. 101. of sea-water were filtered using 47-mm HA-type ‘Millipore’ filters with 0-45u pore size. After filtering and chemical separation, the samples were analysed for y-emitting radionuclides. The Christmas Island samples were analysed also for strontium-90. On March 20, 1963. a 100-1. sarmple was collected off the Oregon coast. This sample was divided into 50-1. aliquots which were analysed by different methods for strontium-90. The particulate matter was separated from the water by centrifugation, rather than byfiltration, using an SS-4 Servall enclosed superspeed centrifuge with the Szent-Gyérgyi continuous-flow system. At 17,000 r.p.m.. the flow-rate was 200 ml./min. This system was about 30 per cent as effective as the ‘Millipore’ filter in removing the particulate matter. Chemical separation. A hydroxide precipitation methed ) ~