SummaryA haemagglutinin present in the haemolymph of the Murray Mussel (Velesunio ambiguus) has been purified by density gradient centrifugation and elution from the specific red cell stroma. The purified haemagglutinin is one of several which behave as a homogeneous group of proteins. Amino acid analyses of these proteins indicate that they lack tyrosine and sulphur‐containing amino acids.