Deoxyribonucleic acid of mammalian cells (L5178Y) in culture was labeled uniformly in one of the following three ways: Incubation with tritiated thymidine for one generation (unifilar labeling), incubation with tritiated thymidine for four generations (bifilar labeling), and incubation with tritiated thymidine for four generations plus one generation without tritiated thymidine (reverse unifilar labeling). The cells were frozen and stored in liquid nitrogen. After various storage times, their survivals were estimated. The survival curves of cells with unifilar labeled DNA were approximately exponential. The survival curve of the cells with bifilar labeled DNA was similar to that of the unfilar labeled cells to around 20% survival. Below 20% survival, the slope of the survival curve was greater by a factor of 2 than that of the unifilar labeled cells.

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