Human marrow stromal cells (MSCs) were isolated from posterior illiac crest marrow aspirates obtained from 17 healthy donors, ages 19–45 years, with no apparent physical disability. First passage hMSCs exhibited growth rates in vitro that varied up to 12-fold between donors. No correlation between growth rate and the age or gender of the donor was evident (P ≤ 0.05). When hMSCs were cultured without passage for eight days (subconfluent cultures) or 22 days (confluent cultures) in the absence of any osteogenic agonists, levels of alkaline phosphatase enzyme activity varied 40-fold and 10-fold, respectively, between donors. When exposed to osteo-inductive media, donor populations also showed dramatic differences in levels of bone-specific gene induction. Collectively, these data demonstrate that hMSC cultures are composed of a heterogeneous mixture of cells at various stages of differentiation and with distinct osteogenic potentials. Differences in both growth rate and ALP activity were evident in hMSC cultures established from multiple aspirates obtained over a six month period from the same donors. Therefore, it appears that cellular heterogeneity produced by the method of harvest is propagated within and among different donor populations during culture expansion in vitro. J. Cell. Biochem. 75:424–436, 1999. © 1999 Wiley-Liss, Inc.