Fertilization in mammals is associated with the generation of intracellular calcium ([Ca²⁺]i) oscillations. The site of, or mechanism(s) utilized by, the sperm to initiate and maintain these Ca²⁺ responses is not known. In this study, we tested the hypothesis that a factor from the sperm is capable, upon release into the oocyte's cytosol, of initiating oscillations. A sperm factor, prepared from porcine semen, was injected into mouse oocytes and bovine eggs that had been loaded with fura‐2 dextran, a fluorescent Ca²⁺ indicator. The resulting Ca²⁺ responses were monitored and compared to those characteristic of each species. Our results show that injection of sperm factor triggered long‐lasting [Ca²⁺]i oscillations, and that the observed patterns were species‐specific. In mouse oocytes, sperm factor‐induced [Ca²⁺]i rises exhibited high frequency, whereas in bovine eggs, Ca²⁺ responses were separated by long intervals. Further characterization of the sperm factor revealed that it was predominantly present in sperm preparations, that it contained a protein moiety, and that it was unlikely to be a protease. The intracellular Ca²⁺ channels/receptors through which the sperm factor‐mediated Ca²⁺ release was investigated by using heparin, a competitive inhibitor of the inositol 1,4,5 trisphosphate receptor (InsP3R), and ryanodine, which binds the ryanodine receptor (RyR). The sperm factor appeared to stimulate InsP3R, at least in mouse oocytes, because sperm factor‐induced oscillations were delayed or blocked in all oocytes by injection of heparin. RyR may be involved in the modulation of these oscillations, since addition of ryanodine modified Ca²⁺ responses to the sperm factor. The present results support the hypothesis that a factor from the sperm is involved in the generation of fertilization associated [Ca²⁺]i oscillations. Mol Reprod Dev 46:176–189, 1997. © 1997 Wiley‐Liss, Inc.