Bone marrow-derived mesenchymal stem cells (BM-MSCs) can differentiate into many lineages. Although the growing interest in BM-MSCs has led to a number of characterization studies, some important biochemical and immunohistochemical properties are still lacking. In this study, morphological and immunophenotypic properties of BM-MSCs were examined in detail. Differentiation potential and growth kinetics of adult rat BM-MSCs were also determined. Immunohistochemistry and RT-PCR results indicated that BM-MSCs expressed myogenic (desmin, myogenin, myosin IIa, and alpha-SMA), neurogenic (gamma-enolase, MAP2a,b, c-fos, nestin, GFAP and beta III tubulin), and osteogenic (osteonectin, osteocalcin, osteopontin, Runx-2, BMP-2, BMP-4 and type I collagen) markers without stimulation towards differentiation. These expression patterns indicated why these cells can easily differentiate into multiple lineages both in vitro and in vivo. Ultrastructural characteristics of rBM-MSCs showed more developed and metabolically active cells.