There is evidence showing that at fertilization the sperm introduces into egg cytoplasm a protein-based cytosolic factor, which serves as the physiological trigger for inducing Ca2+ oscillations in mammalian eggs. Here we show that sperm of nonmammalian vertebrates also contain a cytosolic protein factor that can induce Ca2+ oscillations when introduced into mammalian eggs. We have observed that cytosolic extracts derived from Xenopus or chicken sperm could induce mouse eggs to undergo Ca2+ oscillations similar to those induced by bovine sperm extracts. The factor responsible for inducing Ca2+ oscillations was of high molecular weight and heat- or proteinase K-labile. We show that 0.5 chicken sperm-equivalents or 1-2 Xenopus sperm-equivalents of the extracts had enough activity to trigger Ca2+ oscillations in mouse eggs. Our findings illustrate that although Xenopus, chicken, and mammals are evolutionarily divergent species, the function of the sperm protein factor in triggering Ca2+ oscillations in mammalian eggs appears not to be species specific in vertebrates.