Limiting dilution analysis of stromal progenitors in adult bone marrow. Bone marrow cells were plated at 100 to 500,000 cells/well in a 96-well plate with media containing 25% serum. Wells were sorted for the growth of stromal cells after 2 to 3 weeks of culture. The mean frequency (±SEM) of negative wells was plotted against the logarithm of cells plated into each set of wells. The frequency of clonogenic stromal cells was determined from the inverse of the cell concentration at which 37% of the wells lacked the growth of stromal cells.