Cultured Chinese hamster cells, line M3-1, were exposed to ultraviolet light from a large water-prism monochromator. Wavelengths used ranged from 2300 to 3664 Å. The ability of single cells to form visible colonies was assessed, and survival curves so obtained were found to be qualitatively similar in the wavelength range 2481 to 2923 Å, with all curves having extrapolation numbers of about 4 ± 1. The maximally effective wavelength was found to be in the vicinity of 2700 Å, and the action spectrum exhibited a broad maximum from 2500 to 2800 Å. Fractionated exposures to 2652-Å and 2803-Å light gave similar dependencies of colony-formation survival on the time elapsed between two exposures. Exposure to light (<tex-math>$200\ {\rm ergs}/{\rm mm}^{2}$</tex-math>) of 2537 Å, 2652 Å, and 2803 Å resulted in long lags in cell multiplication, and nonsurviving cells apparently failed to divide. Of the wavelengths used, light of 2652 Å was maximally effective in delaying cell multiplication. The results are interpreted as indicative that the same group of photon-absorbing molecules is responsible for the inhibition of colony development at all wavelengths used and that proteins as well as nucleic acids may be involved.

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