Muscle satellite cells contribute to muscle regeneration. We have used a Pax3 GFP/+ mouse line to directly isolate (Pax3)(green fluorescent protein)–expressing muscle satellite cells, by flow cytometry from adult skeletal muscles, as a homogeneous population of small, nongranular, Pax7+, CD34+, CD45–, Sca1– cells. The flow cytometry parameters thus established enabled us to isolate satellite cells from wild-type muscles. Such cells, grafted into muscles of mdx nu/nu mice, contributed both to fiber repair and to the muscle satellite cell compartment. Expansion of these cells in culture before engraftment reduced their regenerative capacity.