The first event in the mode of action of the fungal phytotoxin fusicoccin is the binding to specific receptors located at the plasma membrane. They are very likely related to the presence of endogenous compounds able to compete with FC for binding to its receptors. Some biochemical properties of these proteins conform to criteria commonly used for receptors. A functional relationship between fusicoccin receptors and the plasma membrane H(+)-ATPase which is assumed to be the final physiological target of fusicoccin, has been recently suggested. New support in favour of this relationship has been sought by studying a model system prepared by insertion into liposomes of preparations from maize tissues of the two proteins. The model system is capable of H(+)-translocation on ATP addition and the translocation is markedly stimulated by fusicoccin. Thus, it represents a promising tool for the understanding of fusicoccin signal transduction at the molecular level, since it can assist in the detection and assay of the transduction chain components contained in the impure protein preparations.