Preliminary results are reported of a programme of alfa-spectroscopy that is being carried out on natural levels of alfa-activity in humans and their environment. The natural fallout mechanism for Pb210 and Po210 leads to a rather high, rainfall-dependent concentration of these nuclides in foliage, and their subsequent uptake into the tissues of grazing animals has been detected. A limited number of samples of human foods and drinking waters has been examined and the presence in them of varying proportions of Ra226 Ra228, Po210 and uranium is reported. In bone and many of the soft tissues of normal, unexposed human subjects, Po210 appears to be responsible for a large part of the total natural activity, while thorium and uranium isotopes, and possibly Pu239, have been detected in normal lungs. Results are also reported of similar measurements on the tissues of two subjects with histories of occupational exposure to uranium ore and radium.