Green algae of the genus Ulva (syn. Enteromorpha) are common, green macroalgae found throughout the world in the upper intertidal zone of seashores and as a fouling organism on a variety of man-made structures including ships' hulls. Adhesion of motile spores is achieved via the secretion of an adhesive, which is present in spores in highly condensed form within membrane-bound vesicles. The adhesive is initially liquid and displays a hydrogel-like behavior on release. It then starts to undergo "curing reactions," becoming progressively less soluble with time in anionic detergents, less sensitive to proteolysis, and less viscoelastic, which suggests that extensive cross-linking occurs. Spores also become progressively more difficult to detach from a surface. However, the nature of this adhesive curing process is totally unknown. In the present article we have tested the hypothesis that thiol cross-linking may be involved. We show that nontoxic concentrations of the thiol-capping reagent (Ellman's reagent) or thiol-reducing agent (dithiothreitol) effectively inhibit the time-dependent development of adhesive spore strength after attachment to a surface. Furthermore, we show by SDS-PAGE immunoblot analysis of extracted adhesive proteins that the major adhesive antigen retains solubility in the presence of these reagents, after release from spores, which suggests that cross-linking had been inhibited.