Frequency of clonogenic stromal progenitors among different populations of CD38⁻, HLA-DR–fetal bone marrow cells. Single cells in each of the four quadrants were sorted into individual wells of a 96 multi-well tissue culture plate with media containing 25% vol/vol serum. The greatest frequency of stromal progenitors was seen in the CDw90⁺ CD34⁻ population, the second most homogeneous population was the CD34⁺ CDw90⁺ cells. Stromal progenitors were largely absent in the CDw90⁻ Thy1⁻ populations.