The tetrapeptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP), a physiological regulator of cell proliferation, has been principally reported as a potent inhibitor of the proliferation of haematopoietic stem cells and progenitors. The purpose of this study was to investigate whether the AcSDKP may directly affect the proliferative potential of human bone marrow mesenchymal stem cells (MSCs) in vitro. We added AcSDKP to the cultures of human bone marrow mononuclear cells and measured the number and average area of MSC colonies. MTT colorimetric assay and mitotic index determination were further used to examine the proliferative state of the third passage MSCs in subcultures with or without the addition of AcSDKP. In addition, we evaluated whether AcSDKP may kill MSCs by the trypan blue dye exclusion test. The results showed that the colony forming capacity, the number of viable cells and the mitotic index were reduced in human bone marrow MSCs cultured in 1x10(-12) mol/L to 1x10(-9) mol/L AcSDKP. Maximum inhibitory activity appeared in 1x10(-11) mol/L of AcSDKP. No difference in percent of living cells was observed between the MSC subcultures with and without the addition of AcSDKP. As a result, AcSDKP within a certain range of concentrations has negatively regulatory effects on the proliferation of human bone marrow MSCs in vitro.