1. The physiological effect upon the eggs of Nereis of homogeneous groups of ß-rays of different velocities is proportional to their ability to ionize air.
2. ß-rays of low velocity produce a greater amount of physiological change than the same number of rays of high velocity.
3. These conclusions are consistent with, but do not prove, the view that the physiological effects of radiations from radium and x-rays are due to the production by them of an ionization of some substance in the egg.