Bovine embryos were surgically collected from the oviducts of superovulated crossbred heifers 48 h postonset of estrus. The one-celled ova were treated with 4'-6'-diamidino-2-phenylindole (DAPI) and observed under ultraviolet light by fluorescence microscopy. Both male and female pronuclei were visualized, identified and subjected to micromanipulation.