Seaweed and 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 seaweed/marine algae. In brown seaweed/algae the ability to take up radionuclides is based on the ability of alginic acid to form insoluble complexes with strontium. In case when dairy products, cereals, fish or meat may be contaminated, the body upptake can be prevented by addition of alginate to food. Alginate samples in which the ratio mannuronic acid/guluronic acid is less than 1.0 are remarkable active, inhibition reaches 89%. The strontium isotope is bound to the alginate and passes through the intestines without being absorbed into the body. Highest inhibition values were found in Laminari, Ascophyllum and Fucus.