Production of transgenic cattle by microinjection of DNA has been difficult and costly. To explore an alternative method, one- to four-cell bovine embryos were exposed to a replication-defective retrovirus by microinjection of retrovirus producer cells into the perivitelline space. Embryos were cultured in vitro for 3-4 days, then transferred to recipient cows for further development. Thirteen of 22 embryos recovered at 15 days gestation and each of four fetuses recovered at 90 days gestation were transgenic. Fetuses harbored between 2 and 12 proviruses, and within each fetus, identical patterns of integration were observed in seven tissues tested. Estimates of the number of proviruses per cell suggested that in three of the four fetuses, most, and possibly all, cells were transgenic. This technique should facilitate application of transgenic technology to cattle and other agriculturally important species.