The 16-kDa N-terminal fragment of human prolactin (16K hPRL) is a potent angiostatic factor that inhibits tumor growth in mouse models. Using microarray experiments, we have dissected how the endothelial-cell genome responds to 16K hPRL treatment. We found 216 genes that show regulation by 16K hPRL, of which a large proportion turned out to be associated with the process of immunity. 16K hPRL induces expression of various chemokines and endothelial adhesion molecules. These expressions, under the control of NF-B, result in an enhanced leukocyte-endothelial cell interaction. Furthermore, analysis of B16-F10 tumor tissues reveals a higher expression of adhesion molecules (ICAM-1, VCAM-1 or E-selectin) in endothelial cells and a significantly higher number of infiltrated leukocytes within the tumor treated with 16K hPRL compare to the untreated ones. In conclusion, this study describes a new anti-tumor mechanism of 16K hPRL. Since cellular immunity against tumor cells is a crucial step in therapy, the discovery that treatment with 16K hPRL overcomes tumor-induced anergy may become important for therapeutic perspectives.