Drug sensitivity and cell cycle parameters of L1210 cells in culture in exponential (105-4 .times. 105 cells/ml) and plateau phase (>8 .times. 105 cells/ml) of growth were compared. The percentage of cells in G1, S and G2 + M [mitosis] for exponential and plateau phase cultures was 24.1, 70.0 and 5.9, and 42.5, 45.6 and 11.9, respectively. These values correspond to those reported previously for early and late L1210 mouse leukemia cells. Cell survival (measured by cloning) after drug exposure showed that actinomycin D, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, chlorozotocin and streptozocin were equally lethal to cells in both phases, and 1-.beta.-D-arabinofuranosylcytosine, high specific-activity (20 Ci/mmol) [3H]thymidine, vincristine, daunomycin, adriamycin and 5-fluorouracil were more lethal to exponential cells than plateau phase cells. The percentage kill by S phase specific agents such as 1-.beta.-D-arabinofuranosylcytosine and [3H]thymidine corresponded well with the percentage of cells in S. 1,3-Bis(2-chloroethyl)-1-nitrosourea was more toxic to plateau cells than exponential cells.