Satellite cells, liberated from the breast muscle of young adult chickens by sequential treatment with collagenase and trypsin, were fractionated by Percoll density centrifugation to remove myofibril fragments and cell debris which otherwise heavily contaminate the preparation. This procedure allowed direct measurements of cell yields (1.5-4 X 10(5) cells/g tissue), plating efficiencies (27-40%) and identification of single cells in culture. In mass cultures, satellite cells gave rise to myotubes on the fifth day, and the progeny of these cells were sequentially passaged several times without losing myogenic traits. In clonal studies, over 90% of the satellite cells gave rise to large clones of which more than 99% were myogenic as demonstrated by the appearance of myotubes. The results obtained with satellite cells differ from observations made using embryonic muscle cell preparation from chicks. In the embryonic system massive formation of myotubes was observed following the third day of culture; sequential subculturing led to overgrowth of fibroblast-like cells following the first passage; and cells gave rise to both small myogenic clones (up to 16 terminally differentiated cells per clone) and non-myogenic clones in addition to large myogenic clones. We conclude that the isolated satellite cells represent a homogeneous cell population and reside in a stem cell compartment.