The aim of this study was to investigate whether bovine spermatozoa possessso-called sperm factor in the cytosolic fraction (CF) which activates bovineoocytes, and whether bovine oocytes matured in vitro are activatedby microinjection of CF extracted from spermatozoa of other species. In thefirst experiment, bovine and human spermatozoa were microinjected intoooplasm of bovine oocytes matured in vitro. Secondly, CF frombovine and human spermatozoa were injected into bovine oocytes. In thethird, CF from human spermatozoa was injected into human unfertilisedoocytes obtained 18-20 h after clinical intracytoplasmic sperm injection(ICSI). We found that microinjection of bovine spermatozoa into bovineoocytes induced oocyte activation, as shown by resumption of meiosis andformation of a female pronucleus, at a significantly higher rate than thebovine sham injection (63.0% vs 43.0%; p < 0.05). On the otherhand, there was no significant difference in activation rate between thehuman sperm injection (35.9%) and the human sham injection(22.9%). Furthermore, microinjection of bovine sperm CF into bovine oocytesinduced oocyte activation at a significantly higher rate than the human CFinjection or sham injection (75.9% vs 14.8%, 20.4%; p < 0.01). Formationof a single female pronucleus and second polar body extrusion was observedin 95.1% of activated oocytes after bovine sperm CF injection. When humansperm CF was injected into human unfertilised oocytes, the activation ratewas significantly higher than following sham injection (76.9% vs 44.0%;p < 0.05). These results indicate the presence of sperm factorin bovine sperm CF which activate bovine oocytes, and suggest the possibilitythat sperm factor has species-specificity at least between bovine and human.