A beta-1,3-glucan-binding protein (beta-GBP) was purified from crayfish plasma, and incubated with laminarin (L), a beta-1,3-glucan. The beta-GBP reacted with laminarin (beta-GBP-L) induced strong spreading and partial degranulation of isolated and separated crayfish granular haemocytes. However, neither the beta-GBP nor laminarin alone induced any changes in the crayfish granular cells. When monolayers of granular haemocytes were incubated with 20-mu-g of beta-GBP-L, more than 82% of the haemocytes were affected. The activity of beta-GBP-L on granular cells was dose-dependent and a plateau was reached at 10-mu-g of beta-GBP-L. The degranulation of crayfish haemocytes induced by beta-GBP-L seemed to occur by a regulated exocytosis, since it was strongly inhibited by specific blockers of this process such as SITS or calmidazolium. Monospecific anti-beta-GBP antibodies also totally blocked the effect of beta-GBP-L on crayfish granular cells. Indirect immunofluorescence staining demonstrated that the beta-GBP-L could bind to the surface of granular cells, whereas beta-GBP did not bind or bound very weakly to the haemocyte surface.