The aim of the present study was to investigate the expression and role of c-Flip long isoform (c-Flip(L)), a known anti-apoptotic protein. No data are currently available on c-Flip(L) in male gonad before puberty; therefore, this study was carried out in prepuberal mouse testis. We investigated pachytene spermatocytes and spermatogonia by immunostaining of testis sections and found a strong and specific expression of c-Flip(L) in pachytene spermatocytes, while spermatogonia expressed very low levels of c-Flip(L). This finding inversely correlated with the caspases activity, which was higher in spermatogonia as compared to pachytene spermatocytes. Other experiments carried out in an organ-culture model revealed that Fas-induced apoptosis was higher in spermatogonia as compared to pachytene spermatocytes. These data suggest that c-Flip(L) may play a role as an anti-apoptotic molecule in the prepuberal mouse testis and open new perspectives in the comprehension of the mechanisms controlling germ cells apoptosis. (c) 2005 Elsevier Inc. All rights reserved.