Since in vitro mammalian cells arrested in the plateau phase or in an extended G1 period, they resemble the Go compartment described by Patt and Quastler for stem cells and by Mendelsohn for tumor cells in the viable but non-growth fraction of some solid tumors. For this reason stationary or plateau cultures may serve as an excellent model system for studies concerning the effects of cancer chemotherapy drugs on cell lethality. In addition, it is important to determine whether cells can repair and recover from drug damage induced during plateau phase, and whether these drugs influence the ability of plateau cells to re-enter the cell division cycle. A better knowledge of the effects of cancer drugs on these viable but non-dividing cell populations has great clinical significance. Since plateau or Go cells are generally not killed by the cell cycle specific drugs, these cells most likely contribute to the repopulation or regrowth of the tumor. Therefore, it is extremely important to identify and characterize drugs which will specifically influence survival, repair and repopulation kinetics of plateau phase cells. That is the purpose of this proposal.