Although it is not yet possible to assess the biological role of natural raddoactivity in human life, it is important to determine its magnitude before any substantial change takes place in its level due to acquired radioactivity. Various techniques for measuring natural body radioactivity are reviewed. The low-level activities encountered require that the radiation detector should combine high sensitivity and stability with a low background. Technniques are described and results presented from measurements of gamma radiation emitted from living subjects; measurements of radioactivity in breath, urine, feces, and ashed specimens; particle tracts in autoradiographs of tissues; determination of potassium-40, radium, and carbon-14 conten of body tissuess dosage to body tissues from natural radioactivitys and a comparison of dosage from acquired body radoactivity with that from natural background. (72 references.) (C.H.)