To investigate molecular effects of anti-sperm autoantibodies on fertilization, we previously established anti-mouse sperm-head auto-monoclonal antibodies (mAbs). Among the mAbs established, one mAb (named Ts4) recognized the sugar moiety of TEX101, a germ cell-marker glycoprotein. In the present study, we examined the immunoreactivity of Ts4 in mouse spermatozoa and fertilized eggs during early embryogenesis to clarify the distribution of the Ts4-reactive antigen in the fertilization process. Similar to TES101 mAb (a specific probe for TEX101), immunopositive staining of Ts4 was observed on spermatocytes, spermatids and spermatozoa within the testis. In contrast to the results obtained with TES101 mAb, Ts4 reacted with the sperm acrosomal region within the cauda epididymis. A Western blot analysis of epididymal sperm extract revealed that Ts4 mainly detected two bands between 100 and 150 kDa, while Ts4 faintly detected a band corresponding to TEX101 at 38 kDa. In addition, Ts4-reactive molecules were observed in the growing early embryo after fertilization. Since Ts4-reactive antigen, potentially a carbohydrate chain, is only observed in reproduction-related areas such as the testis, epididymal sperm-head and early embryo, it is expected to have an effect on fertilization. Therefore, additional studies of this antigen may elucidate the molecular mechanisms underlying the reproductive process.