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When studying the kinetics of DNA synthesis, growth and cell division in Escherichia coli B/r after irradiation with different doses of UV-radiation (254 nm) we could demonstrate, by means of pulse incorporation of 3H-thymidine, a lag in DNA synthesis after the irradiation. The relative rate of the restored DNA synthesis (related to the number of viable cells) was higher than in the non-irradiated culture. After 3 h the rate of DNA synthesis settled at a constant value, which was identical with the control rate up to the "critical dose" of 20 J/m2. The irradiated cell population is heterogenous and contains basically two categories of cells--surviving and non-surviving. Cells of both types contribute to DNA synthesis restored after the lag period to a different extent. During the first hour after the irradiation even the nonviable portion of the population, i.e. cells that do not form colonies but are still penicillin-sensitive, is involved in the DNA synthesis.

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Abstract— Problems of absolute measurement of the dose rate of ultraviolet radiation of germicidal lamps in energetic units were studied. Irradiance at 254 nm generated by three different Pen Ray SC‐1 low‐pressure mercury lamps was measured independently in different laboratories using different instruments: (a) Westinghouse SM‐600 Meter, (b) General Electric Germicidal Meter, and (c) large‐surface thermopile with a Bäckström filter. These lamps were then used as secondary standards of absolute irradiance at wavelength 254 nm and compared with the Latarjet dosimeter and the International Light IL‐254 Germicidal Photometer. Mutual agreement of calibration coefficients of three calibrated Pen Ray SC‐1 lamps was roughly within ±5 per cent error. This calibration uncertainty indicates limits of the usefulness of Pen Ray lamps as standards. A direct radiometric calibration using an FT‐16 Schwarz‐Hilger vacuum thermopile, with interference filter NB‐254 or UVR‐250, was in agreement with the above comparison. On the basis of above radiometric calibration absolute D37 lethal doses were determined equaling 2·7. J/m2 for the bacteriophage T2 and 11·0 J/m2 for the bacteriophage φX‐174, the values being read from exponential survival curves.