Calcium in millimolar concentrations protected mouse erythrocytes from hemolysis caused by Vibrio vulnificus cytolysin without affecting the release of intracellular K+ from the cells. This effect was maximal at 25 mM CaCl2. The protection was not absolute and could be partially overcome by increased concentrations of cytolysin. Calcium failed to block both the binding and oligomer formation of cytolysins on the erythrocyte membrane. After pore formation, the continued presence of calcium is required for the prevention of hemolysis. There was hardly any inflow of calcium into the erythrocytes through pores as measured by 45Ca2+ uptake. The presence of calcium after the abolition of Ca2+ gradient by ionomycin cannot inhibit the hemolysis caused by cytolysin. These results suggest that calcium exerts its major inhibitory effect on V. vulnificus cytolysin-induced hemolysis as an osmotic protectant, and that cytolysin may become an useful tool for permeabilizing cells selectively for small ions such as potassium or sodium while preventing the Ca2+ flow.