It was shown by Bowie and Davidson (1955) that measurement of the beta radioactivity of fossil bones in a mixed assemblage is a useful means of determining their relative ages. Buried bones and teeth adsorb uranium from percolating ground water, so that with the passage of time there is an increase in their radioactivity. There can be no confusion with the decreasing radioactivity of the carbon-14 of residual collagen in fossil bones, for this is far too feeble to be measured by the technique employed here. When fresh bone is tested by the present method no radioactivity can be detected. The uranium content of a fossil bone depends not only on the length of time that it has been buried, but also on the permeability of the matrix, on the hydrological conditions at the site, and on the uranium content of the water coming into contact with the specimen.