Human adipose tissue-derived multipotent adult stem cells are provided, which are characterized by the ability to be maintained in an undifferentiated state for a long period of time by forming spheres and which have high proliferation rates. Also provided are methods for isolating and maintaining the adult stem cells, and methods for differentiating the multipotent adult stem cells into nerve cells, fat cells, cartilage cells, osteogenic cells, muscle cells, endothelial cells, hepatic cells and insulin-releasing pancreatic β-cells. Also provided are cellular therapeutic agents for treating osteoarthritis, osteoporosis, nerve disease, diabetes and for forming breast tissue, which contain differentiated cells or the adult stem cells.