Granulysin is a cytolytic protein present in granules of human natural killer (NK) cells and activated cytotoxic T lymphocytes (CTLs). Granulysin is synthesized as a 15 kD precursor, which is then cleaved at both the amino- and carboxy-termini into a 9 kD activated form. The 9 kD granulysin is released into the intercellular space between target and effector cells via a granule exocytosis pathway. The 9 kD granulysin is a chemoattractant for T lymphocytes, monocytes, and other inflammatory cells and activates the expression of a number of cytokines, including RANTES, MCP-1, MCP-3, MIP-1α, IL-1, IL-6, IL-10 and IFN-α. High serum levels of granulysin are associated with cytotoxic T cell and NK cell activity, and may thus represent a soluble biomarker for host-cellular immune responses. Serum granulysin has also been proposed as a biomarker for the Th1/Th2 balance. At the cellular level, granulysin may serve as a favorable prognostic marker for certain types of cancer.