Lymphocytes are a constituent of normal marrow. Both B and T lymphocytes are derived from bone marrow stem cells. Lymphocytes are found in normal marrow as single cells and in lymphoid aggregates or follicles. Lymphocytes and precursors are particularly prominent in bone marrow from children in which they may account for up to 40% of the bone marrow cells. The development of hematopoetic cells within the bone marrow (BM) occurs in intimate association with cells of the bone marrow microenvironment. This phenotypically diverse population of connective tissue-type cells includes fibroblasts, macrophages, adi-pocytes and endothelial cells and, collectively, represents the stromal tissue of the bone marrow. The presence of myoid cells in human bone marrow has been observed during hemopoiesis in embryonic life, whereas during adult life, it is strictly related to dif­ferent pathologic conditions such as metastatic carcinoma, Hodgkin's disease, hairy cell leukemia and chronic myelo-proliferative diseases. Under normal circumstances, lymphoid cells may constitute up to 20% of the population of nucleated cells in the bone marrow. However, there may be an absolute or a relative increase, the latter due to a reduction in hematopoietic tissue, as in some skeletal areas in advancing age, or in hypoplastic conditions. The aim of this study was to examine the presence, distribution and quantitation of cells expressing a-smooth muscle actin in the stroma of the BM of patients with nodular type b-cell chronic lymphocytic leukemia (B-CLL), patients with autoimmune disorders and embryos (gestational age 15 to 25 weeks). For this reason, we investigated the presence of myoid cells (MCs) in a series of 20 trephine bone marrow biopsies from adult patients and ten fetal specimens of the spine and femur, using a monoclonal antibody recognizing alpha-smooth muscle actin, a con­tractile microfilament expressed exclusively by smooth muscle cells, myofibroblasts and related cells. The results of our study showed that: 1. BM stromal myoid cells represent a distinct subpopulation of reticular cells in the bone marrow, undergoing cytoskeletal remodeling in response to various stimuli (fetuses). 2. The appearance of BM stromal myoid cells is not only seen as a characteristic feature in B-CLL, but is also seen, to a lesser degree, in the stroma of bone marrow in patients with autoimmune disorders. 3. Stromal cells with phenotypic smooth muscle features appear in bone marrow during pathological situations in a manner reminiscent of what occurs during normal development.