Coinjection of unfertilized mouse oocytes with sperm heads and exogenous DNA encoding either a green fluorescent protein (GFP) or β-galactosidase reporter produced 64 to 94 percent transgene-expressing embryos, reflecting DNA–sperm head association before coinjection. Nonselective transfer to surrogate mothers of embryos in the GFP series generated about 20 percent offspring expressing the integrated transgene. These data indicate that exogenous DNA can reproducibly be delivered into an oocyte by microinjected spermatozoa and suggest an adaptable method of transgenesis.