AbstractHCA and HML represent lectins isolated from the red marine algae Hypnea cervicornis and Hypnea musciformis, respectively. Hemagglutination inhibition assays suggest that HML binds GalNAc/Gal substituted with a neutral sugar through 1–3, 1–4, or 1–2 linkages in O‐linked mucin‐type glycans, and Fuc(α1–6)GlcNAc of N‐linked glycoproteins. The specificity of HCA includes the epitopes recognized by HML, although the glycoproteins inhibited distinctly HML and HCA. The agglutinating activity of HCA was inhibited by GalNAc, highlighting the different fine sugar epitope‐recognizing specificity of each algal lectin. The primary structures of HCA (9193±3 Da) and HML (9357±1 Da) were determined by Edman degradation and tandem mass spectrometry of the N‐terminally blocked fragments. Both lectins consist of a mixture of a 90‐residue polypeptide containing seven intrachain disulfide bonds and two disulfide‐bonded subunits generated by cleavage at the bond T50–E51 (HCA) and R50–E51 (HML). The amino acid sequences of HCA and HML display 55% sequence identity (80% similarity) between themselves, but do not show discernible sequence and cysteine spacing pattern similarities with any other known protein structure, indicating that HCA and HML belong to a novel lectin family. Alignment of the amino acid sequence of the two lectins revealed the existence of internal domain duplication, with residues 1–47 and 48–90 corresponding to the N‐ and C‐terminal domains, respectively. The six conserved cysteines in each domain may form three intrachain cysteine linkages, and the unique cysteine residues of the N‐terminal (Cys46) and the C‐terminal (Cys71) domains may form an intersubunit disulfide bond.