The objectives of this study were to compare the fertilization rate of bovine in vitro matured oocytes by in vitro fertilization (IVF) and by microinjection of a single spermatozoon (MI) and to relate these rates with fertility reported for these bulls in artificial breeding. Bull A (Holstein) had a nonreturn rate of 75%. Semen from this bull is routinely used in our standard IVF procedure. Bull B (Ayrshire), used regularly in artificial breeding and related to bull D, had a nonreturn rate of 69.2%. Bull C (Brown Swiss), with a chromosomal translocation and trisomy, achieved a nonreturn rate of 42%. Bull D (Ayrshire) produced nonmotile spermatozoa (SPZ) and had an abnormality described as "tail stump defect." No pregnancies sired by bull D have been reported. Oocytes were either fertilized in vitro by capacitated SPZ or by microinjection of a single immobilized SPZ into the ooplasm. SPZ were treated with 0.1 microM A23187 and used for IVF. For microinjection SPZ were cocultured for 5 h with bovine oviduct epithelial cells (BOEC) and then immobilized by freezing and thawing twice without cryoprotectant. A single batch of killed SPZ (stored at -25 degrees C) was used for all microinjections. All oocytes were cultured in Medium 199 for 22 h at 39 degrees C and subsequently fixed, stained, and examined for evidence of fertilization (i.e., female and male pronucleus formation, SPZ decondensation). Fertilization rates following IVF with semen from bulls A, B, C, and D were 80%, 54%, 1%, and 2%, and following microinjection were 39%, 22%, 21%, and 34%, respectively.