A major bottleneck in the use of adult-derived stem cells for therapeutic applications is the recognized inability to generate sufficient numbers of functional cells. This work demonstrates the existence of adult human mesenchymal progenitor cells (MPCs) that can proliferate, in a cytokine-dependent manner, as individual cells under a non-contact suspension culture condition while maintaining their ability to form functional differentiated mesenchymal cell types, including bone. A two-level factorial design experiment was employed to identify cytokines that had the most potent effect on total cells, colony forming unit-fibroblast (F) and CFU-osteoblast (O) expansion in serum-free conditions. Recombinant human stem cell factor [rhSCF] and interleukin-3 [rhIL3] were shown to support the growth of both CFU-F and CFU-O. Factorial design analysis revealed that IL3 was critical for CFU-F and CFU-O survival. The development of this serum-free suspension culture system promoted the direct examination of the effects of specific endogenous and exogenous factors on the growth of MPCs in suspension. Accordingly, the specific role of IL3 on the expansion MPCs in suspension was examined. Flow cytometric analysis of a sorted CD45- cell population (the cell fraction containing MPCs) revealed the competitive emergence of significant numbers of CD45- CD123+ cells after 21 days of suspension culture in direct response to IL3 and SCF. Cell sorting on the CD45--sorted population demonstrated that a subpopulation of CD45-CD123+ cells had the capacity to give rise to CFU-F and CFU-O throughout culture expansion. The number of CFU-F and CFU-O, from the CD45-CD123+ cell population, increased throughout the study period, suggesting that IL3 interacts with CD123 receptor expressing cells resulting in their proliferation. These studies also revealed that hematopoietic cells may also secrete factor(s) that affect the growth of MPCs, in response to the addition of exogenous cytokines, such as IL3 and SCF. Therefore, we can conclude that MPC expansion in suspension is modulated by hematopoietic cells (via their endogenous secretion of factors) and by the exogenous supplementation of IL3 to the culture medium. Importantly, these results provide a plausible mechanism by which MPC expansion is achieved under non-contact culture conditions. Our approach provides an alternative strategy to expand adult BM-derived non-hematopoietic progenitor cell numbers in a scalable and controllable bioprocess and also provides new insight into, and possibilities to explore, mesenchymal stem/progenitor cell biology.