Previous studies evidenced that cystatin B-like gene is specifically expressed and induced in large circulating coelomic cells following bacterial challenge in the leech Theromyzon tessulatum. In order to understand the role of that cysteine proteinase inhibitor during immune response, we investigated the existence of members of cathepsin family. We cloned a cathepsin L-like gene and studied its tissue distribution. Immunohistochemical studies using anti-cathepsin L and anti-cystatin B antibodies and ultrastructural results demonstrated the presence of three distinct coelomic cell populations: (1) the chloragocytes, which were initially defined as large coelomocytes, (2) the granular amoebocytes and (3) small coelomic cells. Among those cells, while chloragocytes contain cystatin B and cathepsin L, granular amoebocytes contain only cathepsin L and the third cell population contains neither cathepsin nor inhibitor. Finally, results evidenced that cathepsin L immunopositive granular amoebocytes are chemoattracted to the site of injury and phagocyte bacteria.