The horseshoe crab Limulus has a single cell type in the blood, the granular amoebocyte. These cells, which are non-adhesive and non-motile in the blood, transform to an adhesive and motile state following extravasation. In the present study, the events of transformation were studied by time-lapse microcinematography of cells plated on to glass. Following attachment of cells to the glass, the cells protrude filopods and engage in active motility. Motile cells show 2 convertible forms; a contracted form and a reversibly flattened form. Eventually the motile cells undergo irreversible flattening which is accompanied by spontaneous degranulation. The shapes of irreversibly spread cells are influenced by the arrangement of cell-substrate contacts established prior to degranulation, and by the pattern of contact of domains of granule-derived membrane introduced into the cell surface during granule exocytosis and their subsequent endoctyosis by the cell. The clottable protein released by spontaneous degranulation of the amoebocytes undergoes gelation in the absence of bacterial endotoxin. The ability to undergo spontaneous gelation stands in marked contrast to the absolute requirement for endotoxin for gelation of the clottable protein present in 'Limulus amoebocyte lysate', a commercially valuable preparation obtained from amoebocytes used to detect endotoxin.