Human marrow stromal cell (hMSCs) were recently shown to expand rapidly in culture when plated at a low density of approximately 3 cells/cm2. Low-density plating promoted proliferation of small recycling stem (RS) cells that appeared to be the most multipotent cells in the cultures. Here we demonstrated that MSCs from rat bone marrow (rMSCs) are even more sensitive to low-density plating than hMSCs. When plated at approximately 2 cells/cm2, the cells expanded over 4,000-fold in 12 days, over twice the maximal rate observed with hMSCs. Analysis by fluorescence-activated cell sorter demonstrated that rMSCs had the same heterogeneity seen with hMSCs in that the cultures contained both small rapidly RS cells and much larger mature cells (mMSCs). The rat mMSCs differed from human mMSCs in that they regenerated RS cells in culture. Also, after low-density plating, colonies of rMSCs expanded into confluent cultures, whereas colonies of hMSCs did not.