The variety of metabolic and morphologic characteristics found in microorganisms offers an opportunity to study the influence of these factors on cellular sensitivity to X-rays that has not been fully utilized. A review of the literature for bacteria showed that the survival curve and LDs0 had been determined accurately for members of only nine genera: Achromobacter, Aerobacter, Escherichia, Micrococcus, Pseudomonas, SalmoneUa, Sarcina, Serratia, and Shigella. Seven of these are gram-negative rods MATERI AND METHODS Microbiological. The strains employed and their sources were as follows: Azotobacter agile strain M.B.4.4, Rhodopseudomonas spheroides strain ATH 2.4.1 (C. B. van Niel, Stanford Univ.), Escherichia coli strain W-1485 (E. M. Lederberg, Univ. of Wisconsin), Pseudomonas fluorescens strain A.3.12 (R. Y. Stanier, Univ. of California, Berkeley), Saccharomyces cerevisae, haploid, strain 93.1C (a) and diploid strain 93 (C. A. Tobias, Univ. of California, Berkeley). In the case of Saccharomyce8, the original Lindegren strain numbers are used. Zirkle and Tobias (1953) called the haploid and diploid strains SC-7 and SC-6, respectively. E. coli, P. fluorescens, and R. spheroides were routinely grown and plated on yeast agar containing NH4Cl, 0.1 per cent; K2HP04, 0.1 per cent; MgSO4.7H20, 0.05 per cent; yeast extract 571
