A rat IgM monoclonal antibody has been developed which recognizes a mouse germ cell nuclear antigen (GCNA1). GCNA1 is present in prospermatogonia (gonocytes) in males and in oogonia and oocytes of females within the gonadal ridge from Embryonic Day 11.5 onward, but rarely in primordial germ cells prior to their arrival at the gonadal ridge. Immunolocalization demonstrates that GCNA1 is abundant in nuclei of spermatogonia and early spermatocytes, but decreases during subsequent spermatocyte and round spermatid development, and is not detected beyond step 10 elongating spermatids. The antigen is ∼80-110 kDa on immunoblots of isolated pachytene spermatocytes and round spermatids. However, GCNA1 appears to be absent from sperm in the epididymis and vas deferens, Sertoli cells, TM3 cells (Leydig-like) and TM4 cells (Sertoli-like), lung, liver, kidney, spleen, heart, skin, brain, epididymis, and ovary. GCNA1 is present in prepuberal male mice (Days 2-14) in all stages of prespermatogonial and spermatogonial development. It is also present in oocytes of neonatal females until Postpartum Day 12. GCNA1 is first lost from oocytes in the medulla of the ovary as they arrest at the dictyate stage and gain a layer of granulosa cells. In addition, the antigen is present in moderate amounts in F9 embryonal carcinoma cells and SCC-PSA1 pluripotent teratocarcinoma cells. Thus, GCNA1 serves as a common marker of the germ cell lineage in male and female mice after primordial germ cells arrive in the gonadal ridge until they reach the diplotene/dictyate stage of the first meiotic division.