Maspin is a serine protease inhibitor with anti-tumor activity, including inhibition of tumor growth, angiogenesis, invasion, motility, and metastasis. Normal mammary and prostate cells express maspin at high levels. In contrast, breast and prostate cancer tissue samples and cell lines exhibit reduced or no expression of the maspin transcript. Previously we have demonstrated that introduction of an intact chromosome 18 into the bone-derived metastatic prostate cancer cell line, PC-3, resulted in reduced in vitro growth and in vivo metastatic potential. The goal of this study was to determine whether maspin is the tumor/metastasis suppressor on chromosome 18 responsible for this phenotype. To investigate whether maspin, when produced at endogenous levels, is capable of inhibiting metastasis to bone we transfected a bacterial artificial chromosome (BAC) genomic clone containing the maspin gene into PC-3 cells that aggressively metastasize to bone in an animal model. The BAC transfected PC-3 cells exhibited an in vitro phenotype consistent with maspin acting as a tumor suppressor. Analysis of the PC-3 maspin transfectants in an in vivo bone metastasis assay resulted in significant reduction of the number and severity of skeletal metastasis, compared with parental PC-3 cells. However, maspin had no effect on the ability of PC-3 cells to metastasize to extra-skeletal sites in this model. These results indicate that maspin expression likely plays a role in reducing the tumor cell's ability to seed to bone or in inhibition of growth in the bone microenvironment. However, it does not affect the ability to metastasize to distant sites.