A novel C-type lectin designated Manila clam lectin 3 (MCL3), with a molecular weight of 17.380kDa, was identified among haemocyte expressed sequence tags of Perkinsus olseni-infected Manila clams. MCL3 was expressed in Escherichia coli M15 cells and purified with a Ni-NTA His-binding resin matrix. MCL3 agglutinated rabbit erythrocytes in the presence of Ca(+2). MCL3-induced agglutination was partially inhibited by GalNAc, Man, lactose, and raffinose, whereas the polysaccharides bovine mucin type II and Candida mannan completely inhibited agglutination. MCL3 was expressed in the haemocytes of Manila clams 3 days after infection with P. olseni and 1 day after infection with Vibrio tapetis. Based on real-time PCR analysis, mRNA transcripts of MCL3 were present in the adductor, foot, gill, mantle, palp, and siphon of Perkinsus-infected Manila clams. Furthermore, MCL3 was detected in the foot, gill, mantle, gonad, digestive gland, gonad connective tissue, intestine, and stomach by immunohistochemical localization.