The nature of the hemopoietic stem cell compartment has been the subject of much controversy. Data are presented to support the concept of a 'continuum' model of the stem cell compartment. The important characteristics of this model are that within the continuum there are cells with varying proliferative capacities. As cells move through the compartment, their proliferative capacity becomes more limited, their likelihood to be in cycle increases, and their commitment to a specific differentiated pathway increases. Experiments with busulfan, cyclophosphamide, 5-flurouracil, and BCNU demonstrate defects in proliferative potential of the surviving CFU-S population. These defects persist throughout the life of the animal without any evidence of recovery. The clinical implications of late stem cell failure may be important as a consideration in the use of cytotoxic agents.