In previous studies (Chen, W. Y. et al., Clin. Cancer Res., 5:3583-3593, 1999; Chen, N Y. et al., Int. J. Oncol., 20:813-818, 2002), we have demonstrated the ability of the human prolactin (hPRL) antagonist, G129R, to inhibit human breast cancer cell proliferation in vitro and to slow the growth rate of tumors in mice. We further revealed that the possible mechanisms of G129R antitumor effects act through the induction of apoptosis via the regulation of bcl-2 gene expression. It has been established that to sustain tumor growth, it is necessary for the development of a network of blood vessels to bring in nutrients, a process called angiogenesis. The disruption of angiogenesis has been proven to be an effective strategy to cause regression of certain tumors. One of the best-studied angiogenesis inhibitors is endostatin, which acts through the inhibition of endothelial cells. In this study, we combine the anti-breast tumor effects of G129R and the antiangiogenic effects of endostatin by creating a novel fusion protein (G129R-endostatin) specifically for breast cancer therapy. The data presented here demonstrated that this novel fusion protein was able to bind to the PRL receptor (PRLR) on T-47D human breast cancer cells and inhibit the signal transduction induced by PRL. At the same time, G129R-endostatin inhibited human umbilical vein endothelial cell (HUVEC) proliferation and disrupted the formation of endothelial tube structures with potency similar to that of endostatin. More importantly, the therapeutic efficacy of G129R-endostatin was confirmed using a mouse breast cancer cell line 4T1 in vivo. G129R-endostatin has a significantly prolonged serum half-life as compared with that of G129R or endostatin alone, and exhibited greater tumor inhibitory effects than G129R and endostatin individually or in combination. Taken together, these data demonstrate the dual therapeutic effects of G129R-endostatin, and suggests that this fusion protein has great promise as a novel anti-breast cancer agent.