The Orchidaceae species Listera ovata and Epipactis helleborine contain two types of mannose-binding proteins. Using a combination of affinity chromatography on mannose-Sepharose-4B and ion exchange chromatography on a Mono-S column eight different mannose-binding proteins were isolated from the leaves of Listera ovata. Whereas seven of these mannose-binding proteins have agglutination activity and occur as dimers composed of lectin subunits of 11-13 kDa, the eighth mannose-binding protein is a monomer of 14 kDa devoid of agglutination activity. Moreover, the monomeric mannose-binding protein does not react with an antiserum raised against the dimeric lectin and, in contrast to the lectins, is completely inactive when tested for antiretroviral activity against human immunodeficiency virus type 1 and type 2. Mannose-binding proteins with similar properties were also found in the leaves of Epipactis helleborine. However, in contrast to Listera only one lectin was found in Epipactis. Despite the obvious differences in molecular structure and biological activities molecular cloning of different mannose-binding proteins from Listera and Epipactis has shown that these proteins are related and some parts of the sequences show a high degree of sequence homology indicating that they have been conserved through evolution.