The aim of this study was to follow the changes of estrogen treatment on osteoblastic MBA-15 cells derived from marrow stromal origin. Following exposure to estrogen, the cells' patterns of protein synthesis and expression were monitored. The proteins synthesized by MBA-15 cells were identified in cell lysate fractionated to soluble proteins (SOL), cytoskeleton (CK), membrane and nuclei, and intermediate filaments (PL) fractions. These cellular fractions of the osteoblastic MBA-15 cell cultures were assayed on SDS-PAGE of total proteins or following radiolabeling of cells by [35S]-Methionine. Changes in cytoskeletal and membrane proteins of the control and treated cells were monitored by these assays. Reduction in expression of tubulin (TUB) and thropomyosin (TM) were observed by western blot analysis and of actin by fluorescein staining. A reduction in expression of an antigen highly expressed by osteogenic cells and detected by MoAb 85/12 was also observed in these cells. These experiments showed reduction in cytoskeletal and other cellular proteins in the stromal osteoblastic MBA-15 cells treated with 17beta-Estradiol in comparison to untreated cells.