Chemical methods were used to isolate 28-year strontium-90, 30-year cesium-137, 280-day cerium144, and 2.4-year prometiuum-147 from as much as 200 liters of sea water. Strontium separation involves precipitation of mixed strontium-calcium carbonates directly and in the presence of (ethylenedinitrilo)tetraacetate ion, precipitation of strontium nitrate, scavenging with ferric hydroxide-barium chromate, and conversion to strontium carbonate. Strontium-90 activity is measured by separating and counting the daughter yttrium-90 activity. Cesium is coprecipitated with dipotassium sodium cobaitinitrite, separated as cesium silicotungstate, converted to the perchlorate, and freed of trace potassium and rubidium by ion exchange. Rare earths are removed by initial coprecipitation on ferric hydroxide. Uranium,protactinium, and thorium are removed by ion exchange; radium and lead are scavenged with barium sulfate. The cerium fraction is obtained in pure form by separation as cerium(IV); promethium, by cation exchange.