LANCL1 (LanC-like protein 1) is related to the bacterial LanC (lanthionine synthetase C) family, which is involved in the biosynthesis of antimicrobial peptides. Highest expression levels of LANCL1 are found in testes and brain, two organs that exist behind blood-tissue barriers. In the mouse, the establishment of an impermeable blood-testis barrier occurs between day 10-16 post natal (pn). Differential display analysis showed that the expression level of LANCL1 mRNA in mouse testes was very low until day 16 pn, but increased gradually from day 16 pn to reach a maximum on days 22-24 pn followed by a slight reduction from day 26 pn to adult animals. Thus, the expression of LANCL1 mRNA is initiated following the establishment of the blood-testis barrier. In situ hybridisation revealed that LANCL1 mRNA was induced in diplotene spermatocytes, which appear for the first time in mouse testes between days 18 and 20 pn, verifying the expression profile determined by differential display. LANCL1 mRNA level remained high in the meiotic division phase and in early round spermatids, but was down regulated in elongating spermatids and it was undetectable in step 9 elongating spermatids in stage IX (as defined by Russel et al., 1990). The steady decrease in expression level from round spermatids in stage I to elongating spermatids in stage IX suggested that LANCL1 mRNA was not transcribed in spermatids. LANCL1 expression in rat testes was initiated already in pachytene spermatocytes in stage IX, but otherwise similar to mouse.