Our work was aimed at researching into the influence of dipeptide (gamma-dGlu-dTrp) "Timodepressin" and this dipeptide-based tripeptides on the colony-forming ability of the irradiated in vitro bone marrow and hemopoietic stem cells of the normal organism. Also studied was the effect of various doses (1-1000 microg/kg) of one oftripeptides (dAla-gammadGlu-dTrp) on the output of exogenous splenic colonies in the case of its introduction 48 hours before irradiation. It is shown that the mode of influence of the preparations produced on the basis ofdipeptides dGlu-dTrp and gamma-dGlu-dTrp on the initial stages ofa hemopoiesis in the normal and irradiated organism depends on the nature of the additional amino-acid residue and its optical orientation.