Various microorganisms and bacteriophages that have been inactivated or rendered nonviable by ultraviolet irradiation can be reactivated to a very high degree by exposure to visible light (Kelner, 1949a,b; Dulbecco, 1949; Novick and Szilard, 1949; Johnson, Flagler, and Blum, 1950) . Kelner investigated the photoreactivation of four species of microorganisms including Escherichia coli, strain B/r. In the study of the photoreactivation phenomenon, E. coli strain B, as well as strain B/r, was

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... The former reactivates with visible light as does the latter, but in addition a similar reactivation of B cells can be accomplished by heat in the absence of visible light (Anderson, 1949a,b; Stein and Meutzner, 1950) . The present paper is a more extended report on this phenomenon. METHODS The cultures used in these studies, strain B and its radiation-resistant mutant B/r, were grown for 16 to 18 hours at 37 C under aeration, in standard Difco nutrient broth. Suspensions to be irradiated were prepared by dilution of the culture into a non-ultraviolet-absorbing buffer to give cell concentrations of 1 to 2 X 107 organisms per ml. Irradiation was carried out using 5-ml portions contained in a 50-ml rotating quartz flask. Two 15-watt G.E. germicidal lamps served as a source of ultraviolet. The lamps were horizontally mounted in a ventilated housing with their centers directly over a 2.5-inch-square opening through which the irradiation was directed upon the quartz flask. The flask was adjusted at a distance to irradiate the suspension at a rate of approximately 700 ergs per second per cm2. After irradiation, samples were removed from the flask and appropriately diluted in sterile buffer solution. Five-hundredths ml of the final suspension were placed on the surface of each of four nutrient agar plates and spread with a glass spreader. As a standard procedure all plates were preincubated overnight, removed just prior to use, and returned to the incubator immediately after having been spread. All seeded plates were placed in a single layer on the incubator shelves to ensure a minimum time to reach temperature equilibrium and to obtain uniform temperature for all plates at any incubation temperature. The seeded plates were incubated at 30, 37, and 40 C in the dark, or in medium light intensity at 37 C. Plates at 30 C were incubated for 48 to 72 hours, whereas