The fusicoccin-binding protein was solubilised from purified oat root plasma membranes. The solubilised protein retained full binding activity, provided that protease inhibitors were included. Sodium fluoride reduced the high-affinity [3H]fusicoccin binding to almost zero in a concentrationdependent way, with an optimum at approximately 20 mM sodium fluoride. The presence of magnesium (> 100 pM) was required for the inhibitory action of fluoride, whereas addition of low amounts of aluminium (25 pM It is concluded that the inhibitory effect of fluoride upon the binding of fusicoccin is indirect and mediated through activated GTP-binding proteins. A hypothesis on the mechanism of fusicoccin action is presented wherein the fusicoccin-binding protein is one component of a signal-transduction chain, two or more steps downstream of a heterotrimeric GTP-binding protein.The fungal toxin fusicoccin has evoked great interest amongst plant physiologists, ever since its discovery was reported (Ballio et al., 1964). The toxin affects all higher plant cells tested thus far and typical responses of these cells are hyperpolarization of the membrane potential, increased H' efflux and K+ influx followed by elongation growth of the cell. Initially, it was suggested that fusicoccin might interfere with the signal-transduction pathway of hormones like auxin and abscisic acid. Recent studies by Hager et al. (1991) have cast doubts on a similar mode of action of auxin and fusicoccin; indole-3-acetic acid did promote the incorporation of H+-ATPase molecules into the plasma membrane by vesicle fusion while fusicoccin had no measurable effect. Fusicoccin might act through interference with regulatory mechanisms of the H+ pump and reports about the presence of endogenous fusicoccin-like ligands in the plant (Aducci et al., 1980),