CXCL16, a newly discovered CXC chemokine, exists both in a transmembrane and a soluble form. Membrane-bound CXCL16 is expressed by antigen-presenting cells such as monocytes, macrophages, B cells, and dendritic cells in the T cell zone of lymph nodes. Soluble CXCL16 can be generated by constitutive cleavage from the cell membrane and further enhanced by cell stimulation with phorbol esters. Soluble CXCL16 has been shown to induce chemotaxis of Th1, Tc1, and natural killer cells, which express the functional CXCR6 receptor. CXCL16 has also been reported as a novel angiogenic factor for human umbilical vein endothelial cells. Moreover, CXCL16 cDNA was shown to be identical to a novel scavenger receptor that binds phosphatidylserine and oxidized lipoprotein. Recently, expression of CXCL16 and/or CXCR6 was shown in nasopharyngeal carcinomas, gliomas, and rectal cancer. Through a cytokine antibody array, we reported that CXCL16 protein production was increased in aggressive prostate cancer cells compared with the less aggressive prostate cancer cells or benign prostate cells.