MBA-15, a marrow stromal-derived cell line, was shown to express an estrogen receptor. This finding was confirmed byin situhybridization and receptor binding assay. An exposure to estrogen (10−12–10−6m) in a dose response manner resulted in a decrease of cell proliferation as measured by MTT assay. Cell function was measured by enzymatic activities of two osteoblastic markers, CD10/NEP and alkaline phosphatase. These enzymatic activities were elevated following the estrogen treatment. This model enabled direct evaluation of the estrogen effect on stromal osteoblast cells.