There is considerable interest in the biology and therapeutic potential of adult stem cells from bone marrow stroma, variously referred to as mesenchymal stem cells or marrow stromal cells (MSCs). Human MSCs can expand rapidly in culture, but the rate of expansion and the yields of multipotential progenitors are inversely related to the plating density and incubation time of each passage. We have defined conditions for optimizing the yields of cultures enriched for early progenitors. Also, we developed a simple method for assessing the quality of the cultures by phase-contrast microscopy and image analysis or by forward light scatter in a flow cytometer. The cells expanded most rapidly on day 4 after plating, with a minimum average doubling time of about 10 hours for cells initially plated at 10 or 50 cells/cm2. After plating the cells at 1 to 1,000 cells/cm2, the cultures underwent a time-dependent transition from early progenitors, defined as thin, spindle-shaped cells (RS-1A), to wider, spindle-shaped cells (RS-1B), and to still wider, spindle-shaped cells (RS-1C). Assays for adipogenesis demonstrated that the adipogenic potential of cultures was directly related to their ability to generate single-cell-derived colonies and their enrichment for RS-1A cells. In contrast, cultures enriched for RS-1B cells showed the greatest potential to differentiate into cartilage in a serum-free system. The results indicate that, when preparing cultures of human MSCs, it is necessary to compromise between conditions that provide the highest overall yields and those that provide the highest content of early progenitor cells.