We have assayed the capacity of the fungal toxin fusicoccin to induce some of its characteristic effects (acidification of the medium, stimulation of K(+), and of 3-O-methyl-D-glucose uptake) in cell suspensions of Parthenocissus tricuspidata (Siebold et Zucc.) Planchon, Acer pseudoplatanus L. and Oryza sativa L., and in protoplast suspensions prepared from leaves of Nicotiana tabacum L. and Spinacia oleracea L. or from cultures of P. tricuspidata. Evidence is presented showing that all tested biological materials respond to the addition of fusicoccin. The observation that the toxin is also active on protoplasts indicates that the cell wall is not involved in the mechanism of action of fusicoccin.