Several groups had reported the protective effect of infused Bone Marrow Derived Mesenchymal Stem Cells (BMDMSC) but there is no direct evidence that endogenous BMDMSC have an active role during the injury process. We propose the use of parabiosis to study cell trafficking. Parabiosis, the surgical attachment of two distinguishable mice, establishes a common circulation between the parabionts and therefore creates chimeras without immunosuppression. Two C57BL/6 mice, one GFP+ and the other GFP‐, were surgically connected creating a common circulation. After recovery, the GFP‐ mouse was treated with a single dose of bleomycin or saline intratracheally. Three, 7 and 14 days later, the lungs from the GFP‐ mouse were harvested, cell suspensions made and flow cytometry analysis performed to determine the expression of GFP+ BMDMSC in the lung. We observed an increase in CD45+ GFP cells that peak at day 14 opposite to what is observed with the CD45‐ that peak at day 3. BMDMSC were approximately 0.05% in the control mouse lung. After bleomycin treatment, the levels BMDMSC significantly increased at days 7 (2.0 fold, p<0.05) and 14 (6.5 fold, p<0.01). In addition we observed a decrease in the severity in the lung injury that can be explained by the additive effect of having two bone marrows to supply BMDMSC. We conclude that mobilization of BMDMSC is a sequential set of events during the process of repair