Bovine oocytes removed from 3-5 mm follicles were matured in vitro for 22-24 h. The zonae pellucidae were then dissolved by pronase and the eggs were transferred to droplets of preincubated bovine epididymal spermatozoa. Of 575 oocytes, 510 (88.7%) were fertilized and in 261 (45.4%) of them normal male and female pronuclei were present. In the rest male pronucleus formations were arrested or were undetermined (9.7%). Approximately half of the oocytes were fertilized by greater than or equal to 2 spermatozoa. In some of the polyspermic cells male pronuclei of normal size were accompanied by less advanced stages, including sperm heads at the beginning of decondensation. We conclude that a cytoplasmic substance responsible for male pronucleus formation is present in about 50% of randomly selected bovine oocytes.