Putative myogenic and endothelial (myo-endothelial) cell progenitors were identified in the interstitial spaces of murine skeletal muscle by immunohistochemistry and immunoelectron microscopy using CD34 antigen. Enzymatically isolated cells were characterized by fluorescence-activated cell sorting on the basis of cell surface antigen expression, and were sorted as a CD34⁺ and CD45⁻ fraction. Cells in this fraction were ∼94% positive for Sca-1, and mostly negative (<3% positive) for CD14, 31, 49, 144, c-kit, and FLK-1. The CD34⁺/45⁻ cells formed colonies in clonal cell cultures and colony-forming units displayed the potential to differentiate into adipocytes, endothelial, and myogenic cells. The CD34⁺/45⁻ cells fully differentiated into vascular endothelial cells and skeletal muscle fibers in vivo after transplantation. Immediately after sorting, CD34⁺/45⁻ cells expressed only c-met mRNA, and did not express any other myogenic cell-related markers such as MyoD, myf-5, myf-6, myogenin, M-cadherin, Pax-3, and Pax-7. However, after 3 d of culture, these cells expressed mRNA for all myogenic markers. CD34⁺/45⁻ cells were distinct from satellite cells, as they expressed Bcrp1/ABCG2 gene mRNA . These findings suggest that myo-endothelial progenitors reside in the interstitial spaces of mammalian skeletal muscles, and that they can potentially contribute to postnatal skeletal muscle growth.