The specific binding of concanavalin A (Con A) to semigranular (SG) and granular (G) cells of the crayfishes Pacifastacus leniusculus and Astacus astacus was shown by in vitro agglutination of fixed haemocytes and by direct labelling with fluorescein isothiocyanate-conjugated Con A (FITC-Con A). When non-fixed cell monolayers from both species of crayfish were treated with FITC-Con A, capping of G cells was observed and nearly all of the SG cells showed patching. The percentage of cap-forming G cells from Psorospermium haeckelii-infected A. astacus was clearly higher than that of non-infected crayfish. Also in the signal crayfish P. leniusculus which carries the crayfish plague parasite Aphanomyces astaci within the cuticle, a high percentage of cap-forming G cells was present. In A. astacus which received a Zymosan injection, a higher percentage of cap formation of G cells was observed compared with that of control crayfish. These results show that the presence of parasites alters the binding pattern of Con A to the haemocyte surface and this altered binding pattern suggests that these cells have become activated for cellular defence reactions within the host haemolymph.