say that to evaluate hazard, it is equally important to know both the amount and the species of radioisotope present. in MPC values as given in Table 1 The variation (the difference between sr?° and pmt47 is greater than one million) points out the futility of estimating hazard without knowing the radioisotopes present, a practice that has been followed on occasion in the past. Although it was stated above that most of the observations on fallout in the ocean have been of close-in fallout, the Woods Hole Oceanographic Institution is studying the distribution of five fallout radioisotopes--sr°”, in the Atlantic Ocean, spi25, Cc 3t 37 cel44 and pm tt? __ an area far removed from close-in fallout. Because the amount of these radioisotopes in Atlantic Ocean waters is minute, large samples and special separation methods are required and the number of samples processed is relatively small. The minute weight of fallout that is added to the ocean can be determined by calculating the amount of an isotope in terms of weight units rather than in units of radioactivity. Using sr” as an example and assuming that the radioactivity from sr?? is 30 disintegrations per minute per 100 liters of water--the highest value found in the Atlantic (Bowen and Sugihara, 1957)-- the amount of sr?° in the 100 liters of water is calculated to be 10713 grams. Since there is one gram of stable strontium in 100 liters of water the dilution of the stable strontium by the radioactive strontium is of the order of one part in ten trillion. Therefore,