It is an old and very generally accepted view (e.g., Aschkinass and Caspari, 1901; Redfield and Bright, 1924; Packard, 1931) that the biological effects of x-rays, cathode rays, and radiations from radio-active substances are associated in some way with the ionization they produce in the living material. However, there is as yet no general agreement concerning the exact nature of the part played by the ions.2 In particular, there is still wide divergence of opinion concerning the existence of any effect due to distribution of the ions in the irradiated tissue. This question is especially controversial among workers with x-rays and gamma rays, for the much discussed factor of wavelength, if existent, must operate through differences in distribution of ions.3