Thymosin b4 (Tß4) is the most abundant member of the ß-thymosins and it is known to be the main G-actin sequestering molecule in mammals. Tß4 is a ubiquitous polypeptide, highly conserved across species. Tß4 has various effects on the immune system., including anti-inflammatory activity, down regulation of inflammatory regulators in septic shock and decreased lethality. Furthermore, it accelerates wound healing, and promotes cardiac cell migration, survival and cardiac repair. It also decreases the number of inflammatory polymorphonuclear cells entering a wound and releasing their factors. To better understand the biological activity of Tß4, we isolated and purified Tß4, and determined its effects on cytosolic [Ca2+] and whole-cell currents in single human monocytes, cultured for 1-2 days. After application of 100 nM Tß4, we observed increases in cytosolic Ca2+ and activation of the Ca2+-dependent K+ currents IK and BK. Furthermore, we found that Tß4 increased IK/BK currents when it was induced intracellularly via the patch-pipette. We suggest that Tß4, aside from its well documented effects on actin reorganization, stimulates Ca2+-dependent K+ currents.