pl30, termed on the molecular size, was originally identified as an inositol 1,4,5-trisphosphate [Ins(l,4,5)P₃] binding protein similar to phospholipase C (PLC)-δl, but lacking any phospholipase activity, and recently was renamed PLC-related catalytically inactive protein-1 (PRIP-1). In this study we analysed the effects of the pleckstrin homology domain (PH domain) of PRIP-1 on the Ins(l,4,5)P₃-mediated Ca²⁺ signaling using permeabilized and intact HeLa cells, and found that PRIP-1PH domain had an inhibitory effect on Ca²⁺ signaling. PRIP-1PH domain inhibited the Ins(l,4,5)P₃-mediated Ca²⁺ release from permeabilized cells in a dose-dependent manner, and PLC-δ1PH domain also inhibited but with a higher concentration. When fura-2 loaded HeLa cells transfected with PRIP-1PH domain were stimulated with ATP, it was found that the agonist-induced increase in free Ca²⁺ concentration, observed in control cells, was inhibited in transfected cells. This inhibition was not accompanied by the reduced production of Ins(l,4,5)P₃. These results suggest that Ins(l,4,5)P₃ could be the main PRIP-1PH domain ligand in cells and that this binding has a potential to inhibit Ins(l,4,5)P₃-mediated Ca²⁺ signaling.