Response of mammalian cells following treatment with bleomycin and 1,3-bis(2-chloroethyl)-1-nitrosourea during plateau phase.

One hr of exposure to bleomycin (100 jug/ml) during the plateau phase caused a 10-fold greater decrease in the survival fraction of Chinese hamster ovary cells than was caused by bleomycin treatment during the exponential growth period. One hr of exposure to l,3-bis(2-chloroethyl)-l-nitrosourea (25 Mg/ml) had no effect on the survival fraction of cells treated in the exponential growth period; however, the same dose reduced the survival fraction of cells treated in plateau phase to 0.0002 (this represents a 500-fold decrease in the survival fraction).