A monoclonal antibody (mAb TRA 104) raised against mouse testicular germ cells was able to recognize the nuclei of testicular germ cells at all the stages of differentiation from embryonic gonocytes to spermatids and did not react with any somatic cells. The antigen recognized by mAb TRA 104 was exclusively present in testicular extracts. The molecular weights and isoelectric point (pI) of the antigens determined by Western blotting analysis were 60-110 kDa and 7.2, respectively. This antigen(s) is referred to as a germ cell-specific nuclear antigen(s) (GENA) since GENA was first detected specifically in the genital ridge at around 12 days of gestation by Western blotting analysis. In the testis, the expression increased gradually until adulthood whereas it was lost in the ovary by postpartum day 5. Thus, GENA is a molecule(s) exclusively present in the nuclei of germ cells and may be a useful marker with which to study the mechanism of germ cell development and differentiation at the molecular level.