Thymosin beta 4 (T beta 4), which is the major G-actin sequestering protein, has been shown to have ubiquitous distribution and multiple biological activities. However, T beta 4's functions in relation to natural killer(NK) cells are still unknown. In this study, we show that synthetic T beta 4 peptide increases NK cell cytotoxicity mediated by intercellular adhesion molecule-1 (ICAM-1) through the secretion of cytolytic granules to target cells. This suggests that T beta 4 is a key activator of NK cell cytotoxicity.