Rapid and complete vascularization of ischemic tissues and thick engineered tissues is likely to require vasculogenesis. Therefore, the search for clinically relevant sources of vasculogenic cells and the subsequent development of experimental models of vasculogenesis is of utmost importance. Here, we describe a methodology adapted from the Matrigel plug assay to deliver human blood-derived endothelial progenitor cells (EPCs) and mature smooth muscle cells (SMCs) subcutaneously into immunodeficient mice. One week after implantation, an extensive microvascular network composed of the human EPCs and SMCs is formed within the Matrigel. The presence of human EPC-lined lumens containing host erythrocytes can be seen throughout the implants indicating not only the formation (de novo) of a vascular network, but also the development of functional anastomoses with the host circulatory system. This is a very versatile assay that allows (1) dialing the final microvessel density by varying either the total number of cells in the original cell suspension or the ratio between EPCs and SMCs, (2) studying the effect of substituting another type of perivascular cell for mature SMCs or another type of endothelial cell, (3) tracking each of the implanted cell types by labeling (e.g., GFP tagging) prior to implantation, and (4) studying the effect of genetically modifying the cells prior to implantation. Additionally, this assay is relatively simple to perform and it does not require an incision or surgical procedure. This murine model of human vasculogenesis is ideally suited for studies on the cellular and molecular components of microvessel development, pathologic neovascular responses, and for the development and investigation of strategies to enhance neovascularization of engineered human tissues and organs.