Haemagglutinins have been isolated from two red marine algae Bryothamnion seaforthii and B. triquetrum, which were harvested from the Caribbean Sea, Falcon State, Venezuela. The mass spectra of the haemagglutinins isolated from B. seaforthii showed five monomer peaks (9057.4–9088.8 Da) and those from B. triquetrum showed three peaks (8981.5–8990.2 Da). Isoelectric focusing, under denaturing conditions, gave rise to multiple bands for the haemagglutinins of both species but only one band was observed under native conditions. Gel permeation experiments suggest that the monomer haemagglutinins can readily associate to form dimers, trimers and tetramers. The haemagglutinins agglutinate papainized human A, B and O erythrocytes; the activity is inhibited by glycoconjugates, but not by simple sugars, and is maintained after heating to temperatures up to 90°C. The amino acid sequence of the haemagglutinin from B. seaforthii has been determined together with a partial sequence for that from B. triquetrum. The isolation, and some characterization, of haemagglutinins from B. seaforthii and B. triquetrum, collected from the Ceara coast of Brazil, have previously been reported. Some differences and similarities of these macromolecules obtained from the two different locations are discussed.