From these primary fission products, about 100 other radionuclides may be formed, some existing only for microseconds and others for thousands of years. The radionuclides of significance to biologists are those that exist long enough—no matter how brief the time—to have an impact on a biological system. Factors of biological transport and concentration of long-lived radionuclides make efforts to assess possible environmental effects particularly difficult. It has been asserted, for example, that probably every living cell formed since the early 1950s contains some of the radionuclides produced by nuclear testing. No one knows the significance of such a condition, if it indeed exists. It is certain only that some of the long-lived radionuclides already placed in the environment will be detectable there for hundreds of years and hence will continue to provide material for biological studies. Examining Environments [Illustration: _Seeds produced by plants grown in soil of a radioactive waste disposal area pass (in aluminum cups) on moving belt through a radioactivity detector as part of a study of movement of radioisotopes in food chains._] When radioactivity is injected randomly into the atmosphere by a nuclear detonation, biological disposition begins in many ways, each related to the character of the explosion and the environment in which it occurs. Fallout studies thus involve the tracing of mixed fission products in the biosphere and the collection and analysis of thousands of samples of plant and animal tissue, and usually of water and soils, at many successive times. The radiobiologist then attempts to interpret the accumulated evidence of uptake of radionuclides. Some fallout studies may require sampling over large areas of the earth. Other investigations of fallout or of radioisotopes introduced deliberately into controlled field plots may require years of patient observation in small and circumscribed areas. Studies of ocean fallout, for example, have ranged over hundreds of thousands of square miles of open water. The 1955 United States survey of the Western Pacific was conducted by a scientific team aboard a Coast Guard vessel, the _Roger B. Taney_, in a program called Operation Troll. In 7 weeks the team cruised 17,500 miles, making collections of water and marine organisms at 86 ocean stations on a route extending from the Marshall Islands through the Caroline Islands and the Mariana Islands to the Philippines and finally to Tokyo. The expedition took samples of plankton at depths down to 200 meters and water from the surface down to depths of 600, 800, 1000, and 1200 meters. Environmental studies at nuclear test sites or in controlled ecosystems involve not only long-term, periodic sampling of plants and animals but also years of detailed examination of soils, meteorological conditions, and other factors. TERRESTRIAL ECOLOGY RESEARCH