Dependence on Stage of Growth of Mammalian Cell Sensitivity to Near‐uv Irradiation*

The variable sensitivity of exponentially growing mouse myeloma cells (66.2 subclone of MPC11) to monochromatic near‐UV (365 nm) radiation was studied by determination of the relative cloning efficiency (i.e. survival) of cells in soft agar. Maximum sensitivity of the cells to near‐UV light was found during the middle and late stages of exponential growth when the survival was only one‐tenth of that during the early stage of growth. In addition, the shapes of near‐UV survival curves changed with the stage of the cells irradiated. These data indicate that stages of growth can substantially alter the response of mammalian cells to near‐UV radiation. These data further suggest the importance of testing the response of the cells throughout the growth employed in the experimental procedure.