The concentration of cytoplasmic free calcium (Ca²⁺) increases in various stimulated cells in a wave (Ca²⁺ wave) and in periodic transients (Ca²⁺ oscillations). These phenomena are explained by inositol 1,4,5-trisphosphate (IP₃)-induced Ca²⁺ release (IICR) and Ca²⁺-induced Ca²⁺ release (CICR) from separate intracellular stores, but decisive evidence is lacking. A monoclonal antibody to the IP₃ receptor inhibited both IICR and CICR upon injection of IP₃ and Ca²⁺ into hamster eggs, respectively. The antibody completely blocked sperm-induced Ca²⁺ waves and Ca²⁺ oscillations. The results indicate that Ca²⁺ release in fertilized hamster eggs is mediated solely by the IP₃ receptor, and Ca²⁺-sensitized IICR, but not CICR, generates Ca²⁺ waves and Ca²⁺ oscillations.