Sperm-induced activation of mammalian eggs is associated with a transient increase in the concentration of intracellular Ca²⁺. The role of inositol 1,4,5-trisphosphate (IP₃)mediated release of Ca²⁺ from intracellular stores during mouse egg activation was examined in the present study by determining the effects of microinjected monoclonal antibody (mAb) 18A10, which binds to the IP₃ receptor and inhibits IP₃-induced Ca²⁺ release, on endpoints of egg activation following insemination. The antibody inhibited in a concentration-dependent manner the ZP2 to ZP2_f conversion that is involved in the zona pellucida block to polyspermy, as well as the ZP2 to ZP2_f conversion promoted by microinjected IP₃ in non-inseminated eggs. As anticipated, inseminated eggs that had been microinjected with the antibody were polyspermic. In addition, the antibody inhibited the fertilization-associated decrease in H1 kinase activity and pronucleus formation, and the concentration dependence for inhibition of these events was similar to that observed for inhibiting the ZP2 to ZP2_f conversion. Last, the antibody inhibited the fertilization-induced recruitment of maternal mRNAs and post-translational modifications of proteins. In each case, eggs microinjected with the mAb 4C11, which also binds to the IP₃ receptor but does not inhibit IP₃-induced Ca²⁺ release, had no inhibitory effect on fertilization and egg activation. Results of these studies suggest that IP₃-mediated Ca²⁺ release is essential for both early and late events of mouse egg activation.