Fusicoccin (FC) specifically bound to microsomal preparations from maize coleoptiles can be partially solubilised by treatments with Triton X-100, deoxycholate (DOC), SDS, urea, sodium perchlorate and trypsin. In the cases of the treatments with Triton X-100, deoxycholate, percholate or trypsin a consistent fraction of the solubilised FC appears still bound to a macromolecular component. On Sephadex G-200 filtration the perchlorate-solubilised FC-macromolecule complex shows a well-defined peak, corresponding to a molecular weight of approx. 80 000. Acrylamide disc gel electrophoresis of the bound FC-containing fractions from Sephadex chromatography seperates bound FC in a single, narrow peak corresponding to a minor protein peak. These data are interpreted as suggesting that the high-affinity, low-reversibility FC-macromolecule complex is due to the interaction between FC and a single receptor protein, localised at the plasmamembrane, and possibly involved in the physiological activity of the toxin.