Exposure of human corneal limbal epithelial (HCLE) cells to UVB triggers rapid loss of K+ and apoptosis via activation of caspases -9, −8 and −3. It has been shown that preventing loss of intracellular K+ can inhibit apoptosis. The goal of this study was to investigate the effect of K+ on the UVB-induced caspase activity. HCLE cells were exposed to 150 mJ/cm2 UVB, followed by measurement of caspase activity in cell lysates. Caspase activity was measured in the presence and absence of 100 mM K+ in the reaction buffer. UVB-induced activity of caspases -9, −8 and −3 all decreased in the presence of 100 mM K+. These results suggest that a role of high [K+] in the cell is to inhibit caspase activity. Therefore, when cells lose K+ in response to UVB, caspases are activated and cells go into apoptosis. This supports our hypothesis that K+ inhibits caspase activity.