The proliferation kinetics of mouse L-P59 cells irradiated with ultraviolet light (UVL) were analyzed by constructing cell pedigrees from photographs taken with a time-lapse system. After exposure to$35\ {\rm ergs}/{\rm mm}^{2}$, inactivation occurred primarily after a first mitosis, while after$75\ {\rm ergs}/{\rm mm}^{2}$ 48% of the cells became pyknotic or did not enter mitosis. First-division delay was inversely related to the age of the cells at irradiation, but second-division delay was a direct function of the age of the irradiated parental cells. Hence, pronounced delays occurred in all age groups. With successive divisions, generation times in viable clones approached the values of control cells. During postirradiation generations, cell inactivation was frequently preceded by an abnormal mitosis (i.e., multipolar and/or incomplete cleavage); nondividing and pyknotic cells occurred less often. In terms of cell killing (loss of colony-forming ability) G1 cells were the most sensitive and late${\rm S}\text{-}{\rm G}_{2}$ cells the most resistant, in qualitative agreement with the age dependence of first-division delay. After$75\ {\rm ergs}/{\rm mm}^{2}$, 44% of the nonsurvivors were able to undergo one or more mitoses. It was concluded that the effects of UVL, in comparison with x-rays, show some basic similarities as well as differences.

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