Mesenchymal stem cells (MSCs) from bone marrow appear to be an attractive tool for use in tissue engineering and cell-based therapies due to their multipotent capacity. The majority of studies on MSCs have been restricted to the roles of growth factors, cytokines, and hormones. Based on previous reports demonstrating the important roles of amino acids, we sought to evaluate the effect of essential amino acids (EAs) and nonessential amino acids (NEAs) on the proliferation and differentiation of MSCs. The results showed that the EA/NEA compositions during culture could significantly modulate MSC proliferation and differentiation and, especially, that EAs served as a potent positive modulator in the proliferation of MSCs without causing a deficit in the differentiation capacity of the cells. These results will be very useful in the production of MSC-based cell therapy products for use in the field of tissue engineering and regenerative medicine.