The population of the pluripotential hemopoietic stem cells in mice, i. e. those cells forming colonies in the spleens of lethally irradiated mice (colony forming cells CFc) is proliferating relatively slowly. After a partial damage the population regenerates which is achieved by means of the increased proliferation rate. The effect of damage caused by different doses of hydroxyurea or vinblastine on the proliferation of the CFc has been investigated. CFc population was measured in femur bone marrow after grafting the sample of the bone marrow into the lethally irradiated mice recipients (spleen colony method). The proliferation rate was estimated either according to the magnitude of the fraction of cells synthesizing DNA in the S phase of the cell cycle, or according to the sensitivity of the population to the repeated injections of vinblastine. Data showed that even after very minute damage caused by hydroxyurea the stem cells started to proliferate intensively. The effect was dose dependent. The comparable damage caused by vinblastine had a significantly weaker effect on the proliferation of the stem cells. From the results it is concluded that the proliferation response of the pluripotential stem cells depends on two factors: one being the extent of the damage caused to the hemopoietic tissue and the second the position of the killed cells in the cell cycle.