Bone marrow stromal cells (BMSCs) for osteoblast differentiation studies can be obtained by gradient isolation techniques or by directly plating a filtered cell suspension. We compared these two procedures to evaluate whether this step is critical in order to obtain a high number of differentiated colonies. Isolated primary rat BMSCs were cultured in vitro with or without insulin-like growth factor II (IGFII), basic fibroblast growth factor (b-FGF), epidermal growth factor (EGF) or transforming growth factor beta 1 (TGF beta 1), and histochemically and biochemically analysed at different time points. The gradient procedure produced a significantly higher number of colonies capable of osteoblastic differentiation. The growth factors had different effects. In particular, b-FGF and EGF significantly increased the number of Alizarin red S positive colonics, while IGFII and TGF beta I exerted inhibitory effects. Nodules obtained on day 21 showed some alkaline phosphatase positive cells and were Von Kossa-positive. These data demonstrate that more differentiated colonies are obtainable from BMSCs isolated by the gradient procedure.