Frampton, E. W. (The University of Texas M. D. Anderson Hospital and Tumor Institute, Houston), and B. R. Brinkley. Evidence of lysogeny in derivatives of Escherichia coli. J. Bacteriol. 90:446–452. 1965.—Exposure of cultures of several thymine-requiring derivatives of Escherichia coli strain 15 to radiation [X rays or ultraviolet (UV)] resulted in an increased synthesis of ribonucleic acid (RNA) as measured by the incorporation of radioactive uridine. In addition to radiation, mitomycin C (5 or 25 μg/ml) or growth in the absence of thymine also stimulated RNA synthesis in cultures in which the bulk of RNA synthesis was terminated by a shift-down in growth conditions. RNA purified from X-irradiated bacteria was resolved on columns of methylated serum albumin and yielded an elution profile which appeared similar to RNA synthesized by phage-infected bacteria. Electron microscopic examination of cultures induced to lyse by exposure to UV radiation, or by growth in the presence of 5-bromouracil before exposure to radiation (X rays or UV), showed the presence of mostly incomplete phage particles.