X-ray beams of wave lengths lying in the range between 4 A and 0.5 A all kill B. coli in a semilogarithmically linear fashion. Interpreted in terms of the known quantized absorption of X-rays, this means that one absorption of any of these radiations is sufficient to kill. Though death results from a single absorption, only about one hit in four with Ag K and one in sixty with Ag L radiation is deadly. The course of curves constructed from these experimental results suggests that the portion of this bacterium which is essential to its continued life has a total of approximately 0.01 of the cell volume. For copper and harder radiations the biological action of the rays is proportional to their measured air ionization. The same biological change with the softer chromium K. and silver L X-rays seems to require a somewhat more intensely ionizing beam.