The cationic antimicrobial immunomodulatory peptide, KLK (KLKL5KLK), exerts profound membrane interacting properties, impacting on ultrastructure and fluidity. KLK–membrane interactions that lead to these alterations require the ability of the peptide to move into an α-helical conformation. We show that KLK induces an increase of the intracellular Ca2+ concentration in human T24 cells. The effect of KLK is buffer-sensitive, as it is detected when HBSS buffer is used, but not with PBS. This, together with the lack of effect of the middle leucine-to-proline-substituted peptide derivative [KPK (KLKLLPLLKLK)], indicates that it is the conformational propensity rather than the net positive charge that contributes to the effect of KLK on intracellular Ca2+ level of T24 cells. We show that, although KLK slightly stimulates Ca2+ influx into the cell, the bulk increase of Ca2+ levels is due to KLK-induced depletion of intracellular Ca2+ stores. Finally, we demonstrate a KLK-induced switch of PS (phosphatidylserine) from the inner to the outer plasma membrane leaflet that contributes to the onset of early apoptotic changes in these cells.