Marine algae have a high ability to concentrate metal ions from sea water. For this reason they are an outstanding source for trace elements and health-giving as food. Among the ions concentrated are also radionuclides with low concentration factors of 10 to 100 thus normally only at harmless levels. It has been suggested that hazardous or inappropriate local contamination of sea areas could be detected by assaying radioactivity in marine algae. In brown alga the ability to take up radionuclides is bases on the ability of aginic acid to form insoluble complexes with strontium. In cases when dairy products, cereals, fish or meat may be contaminated, the body uptake can be prevented by addition of alginate to the food. Alginate samples in which the ratio mannuronic acid/guluronic acid is less than 1.0 are remarkably active, inhibition reaches 89%. The strontium isotope is bound to the alginate and passes through the intestines without ever entering into the body. Highest inhibition values were found in Macrocycystis, Hedophyllum, Egregia, Laminaria, Costaria, Ascophyllum and Fucus. If the body uptake is not prevented by alginates the strontium is absorbed be the blood and deposited in the sceleton, where it may cause bone tumours or leukaemia. Alginates can even be used to reduce strontium already deposited in the bone. It is supposed that some of this is released slowly, re-secreted, possible through the bile, and then in the intestines bound by alginate and excreted, if alginate treatment is given, otherwise it is re-aborbed by the blood. Thus alginate can serve both as a preventive and a therapeutic measure. The phycocolloids of red algae do no posses this property and within brown algae is great variation. In addition to a high level of guluronate units of the alginate a certain chain length (ca 150 units polyguluronate) is essential for the highest biological activity. Suitable combinations could be looked for within the natural resources, or obtained through partial hydrolysis of the alginates.