In order to test the diversification among satellite cells in man, satellite cells were isolated from human quadriceps and masseter muscles. The growth kinetics and morphological features of these cells were determined in vitro and the expression of the different myosin heavy (embryonic, fetal, fast, and slow) and light chain isoforms was analyzed. In all satellite cell cultures, only the four fast-type light chains (MLC1emb, MLC1F, MLC2F, and MLC3F) were synthesized and no slow myosin light chains were ever detected. However, we found that fused cultures of human satellite cells express both adult fast and slow myosin heavy chains (MHCs), in addition to embryonic and fetal isoforms. In order to determine if distinct fast and slow cell lineages could be detected among the satellite cells, a clonal analysis was carried out on both cell populations. This analysis was first carried out on clonal populations and was confirmed by the analysis of isolated clones. Double-labeling experiments confirmed that all myogenic clones which expressed fast MHC also coexpressed slow MHC. Therefore, we found no evidence for the existence of different fast and slow satellite cell lineages in human postnatal skeletal muscle.