The contribution to environmental radioactivity of radioactivity due to fallout following the explosion of nuclear weapons is reviewed. Mechanisms by which /sup 90/Sr might be expected to enter the human body are discussed in detail. Data from nuclear weapons tests form the basis for the conclusion that the main part of the radioactivity from high-yield weapons dissipates in the stratosphere, but the small but very significant part that falls out within a few hundred miles of the site of the explosion of weapons fired on the surface constitutes a very real hazard. The weapons tests are therefore conducted with great attention to the dangers and every effort made to protect against misadventure. These local precautions should be entirely adequate and the worldwide health hazards from the present rate of testing are considered insignificant.