Type I interferon (IFN) is shown to control the reversible quiescence of a primitive human bone marrow mesenchymal stem cell (MSC) subpopulation. A 24 h pre-treatment of Stro1+/GlycoA- or CD45-/GlycoA- subpopulations with a monoclonal antibody (mAb) against the IFNAR1 chain of the human type I IFN receptor (64G12), or with a polyclonal anti-IFNalpha antibody, resulted in a marked increase in the number of very large colonies (CFU-F >3000 cells) obtained in the presence of low, but necessary, concentrations of bFGF. Over a 2-month culture period, this short activation promoted a faster and greater amplification of mesenchymal progenitors for adipocytes and osteoblasts. Activation correlated with inhibition of STAT1 and STAT2 phosphorylation and of STAT1 nuclear translocation. A non-neutralizing anti-IFNAR1 mAb was ineffective. We demonstrate that control and activated MSCs express ST3GAL3, a sialyltransferase necessary to produce the embryonic antigens SSEA-3 and -4. Interestingly, activated MSC progeny expressed SSEA-3 and -4 at a higher level than control cultures, but this was not correlated with a significant expression of other embryonic markers. As MSCs represent an essential tool in tissue regeneration, the use of 64G12, which rapidly recruits a higher number of primitive cells, might increase amplification safety for cell therapy.