We isolated multipotential mesenchymal stem cells (MSC) from human postnatal dermal tissues. The isolated cells were expanded and maintained for over 100 population doublings with retention of their chromosomal complement and potential for multilineage differentiation. Progeny of cell lines established from a single dermal MSC could be differentiated into adipogenic, osteogenic, and myogenic lineages, consistent with the conclusion that we established a clonal, multipotential, somatic MSC cell line. This study is the first to show potential growth of multipotential human MSC cell lines from a single cell that can be used for the engineering of tissues in vivo. Clonal growth of MSCs presents profound implications in our understanding of differentiation and development, and should provide a valuable resource for tissue repair.