The in vivo role of epidermal growth factor (EGF) is not well defined even though its effects on culture cells were well studied. To understand the developmental, physiological, and pathological roles of EGF, we have generated transgenic mice widely expressing human EGF with the use of the beta-actin promoter. EGF and transforming growth factor alpha (TGFalpha) bind with equal affinity to the EGF receptor, a transmembrane tyrosine kinase, to trigger various biological responses. EGF and TGFalpha signaling are implicated in the development of the reproductive system. EGF also plays a physiological role in reproduction. Removal of the salivary gland in rodents, which reduces circulating EGF, reduces spermatogenesis, which can be corrected by EGF replacement. Here we show that in our transgenic males, only few post-meiosis II gametes were found, and the mice were sterile. This resembles a common cause of infertility in humans. Furthermore, the transgenic males had reduced serum testosterone. Our findings contrast the previous report on transgenic mice overexpressing TGFalpha in testis, which showed normal spermatogenesis. These data suggest that EGF is the active ligand for EGF receptor reported in germ cells, and proper EGF expression is important for completion of spermatogenesis.