Aims: Alveolar epithelial type II cells (ATII) and type I cells make the alveolar barrier, where the gas exchange takes place. Cytolysins, such as streptolysin-o (SLO), secreted by pathogenic bacteria can breach this barrier and help pathogens gain access into deep tissue or in blood circulation. Recent studies suggested a role for Ca²⁺ in cellular resistance to cytolysins. We investigated the response of H441 and primary ATII cells to SLO. Methods: Threshold concentration of cell lysis was determined by measuring propidium iodide (PI) staining. For simultaneous observation of Ca²⁺ signaling and probable cytolysis, Fura-2 was used together with PI. Results: We found that SLO has a Ca²⁺ dependent cell damaging effect. Sublytic concentration of SLO induces diverse kind of Ca²⁺ signals but the predominant one is oscillatory signal where cells oscillate for as long as 20 min. Phospholipase C (PLC) inhibitor U73122 and emptying internal Ca²⁺ stores by thapsigargin, blocks any signal, suggesting that an IP₃ dependent store release mechanism is involved. Whereas when extracellular Ca²⁺ was depleted, oscillation were still observed though with different kinetics, which suggests that SLO can activate Ca²⁺ release independently of extracellular Ca²⁺. Treatment of cells with the fast Ca²⁺ chelator BAPTA completely abolished the Ca²⁺ signal but interestingly slow chelator, EGTA only decreased the amplitude, which demonstrates the influence of extracellular Ca²⁺ influx on the Ca²⁺ signal. Conclusion: We demonstrated for the first time that SLO induces a complex Ca²⁺ response in alveolar epithelial cells which depends on PLC mediated store release. Such a mechanism can potentially be involved in cellular defence mechanism against pathogens and could protect the organism from bacterial invasion. Supported by the DFG, grant D1402 and the Landesstiftung Baden-Württemberg.