The present paper describes the purification and function of a haemagglutinin from the amoebocyte lysate of the horseshoe crab Carcinoscorpius rotundicauda. The purified protein consisted of a single subunit of Mr 24 000 and agglutinated human blood-group-A+ erythrocytes. Its haemagglutinin activity was inhibited by purified lysate, coagulogen, but not by sugars. The haemagglutinin differed immunologically and in activity from the sialic-acid-binding lectin carcinoscorpin present in the haemolymph. It caused aggregation of forma-fixed amoebocytes, and on the basis of this observation its role in cell-cell adhesion is proposed. This new haemagglutinin promotes cell-cell aggregation in amoebocytes in a manner that shares some similarities with thrombospondin-mediated platelet aggregation in vertebrates [Jaffe, Leuang, Nachman, Levin & Moseher (1981) Nature (London) 295, 246-248].

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