body they are known as internal emitters. The hazard from internal emitters is greater than for external emitters, and by a large factor, with the possible exception of the area in the immediate vicinity of and at the time of a nuclear explosion. Radioisotopes are more hazardous in- side the body than outside because sources inside the body are in intimate contact with the body tissues, and they irradiate the body continuously until they are eliminated, which may be a very long time for some radioisotopes. For practical purposes man's concern about radioisotopes in the sea and their effect upon fish is ultimately in the effect upon his welfare. There is considerable aca- demic interest in the exposure of fish to radioactivity, as this information will contribute to the general knowledge of the biological effect of itonizing radiations on animals but there would be little concern if there were no effect upon man. A loss of a fisheries resource or the uptake of radioisotopes by man from fish are effects about which we are concerned. For example, if the numbers of fish that are killed result in a reduction in the harvest of the fishery, then this is an effect upon one of our resources and indirectly upon our welfare. Or, if fish as an item of food