Four different assay methods of bone marrow stem cells have been examined with regard to the kinetic pattern following perturbation of the steady-state system, e.g., by irradiation. Basically, the stem cell assays fall into two categories: those depending on grafting hemopoietic cells into suitably treated recipients, and those in which recovery of the population is allowed in the animal in which the perturbation was produced, without handling the cells. Evidence is accumulating which indicates that in the grafting techniques, a selective loss of stem cells may occur, especially stem cells in cell cycle; hence, in early stages of recovery of the population, unduly low numerical values might be noted. In view of this evidence, the concept of the colony-forming cell may have to be replaced by the concept of the colony-forming property of the stem cell.