The prophenoloxidase activating system, an enzyme cascade present in arthropod blood, has been shown to be involved in defense and recognition reactions. This system is converted to its active form by fungal 1,3-beta-D-glucans through binding to a plasma protein, a 1,3-beta-D-glucan-binding protein. Here the molecular cloning and carbohydrate composition of the 1,3-beta-D-glucan-binding protein from the freshwater crayfish Pacifastacus leniusculus are reported. It is also demonstrated that this protein can act as an opsonin, stimulating phagocytic uptake of yeast particles by isolated blood cells. The deduced amino acid sequence of 1,339 residues shows no significant similarity to proteins with similar functions in other animals such as the mannan-binding and lipopolysaccharide-binding proteins present in mammals. However, a short sequence motif with similarity to the active site of microbial 1,3-1,4-beta-D-glucan 4-glucanohydrolases was found to occur twice in the 1,3-beta-D-glucan-binding protein.