The use of activation analysis for the determination of microgram quantities of Mn and Zn in sea water was investigated.Total Mn and Zn were separated from the other elements of sea water by coprecipitation with iron as hydroxide, and divalcnt Mn and Zn were separated by extraction with diethyldithiocarbamate.Analyses were made from samples of water from different locations and depths of the North Pacific, Atlantic, and the Gulf of Mexico.Evidence indicates a substantial portion of Mn and Zn is held in sea water in a non-dialyzable and non-extractable form.INTRODUC '.lXON Increasing interest in trace elements in sea water makes it desirable to explore the application of sensitive radioactive methods to determine quantities in the micro and sub-microgram range. The presence of a wide variety of elements, over 40, imposes a very stringent requirement on the method of separation of trace elements from interfering elements, especially from the major components (sodium, chloride, magnesium, and bromine) and others which are prcscnt in comparatively large proportions. Induced activity of an element by thermal neutron bombardment and subsequent determination of the radioactive characteristics of a given element is sensitive as well as sclective. Activation analysis for the quantitative determination of Mn is especially favorable because of the high activation cross section of 13 x 1O-24 cm2 and relatively short half-