Carrot cells (Daucus carota) grown in tissue culture are capable of reducing the non-permeable electron acceptor, ferricyanide, with concomitant proton extrusion from the cell. Optimum conditions for transmembrane ferricyanide reduction include a pH of 7.0-7.5 in a medium containing 10 mM each KCl, NaCl and CaCl2. Data are shown to prove that transmembrane ferricyanide reduction is an enzymatic process. It does not depend on the secretion of phenolics from the cell within the time limits of the assay (10 min). The presence of broken cells and cell fragments are excluded on the basis of stimulation or only slight inhibition by mitochondrial inhibitors. However, transmembrane ferricyanide reduction by carrot cells is inhibited about 50% by various glycolysis inhibitors, which are presumed to reduce the internal levels of NADH. Treatment of cells with p-diazoniumbenzenesulfonic acid, a non-permeant membrane modifying agent, also inhibits transmembrane ferricyanide reduction more than 90%. The data presented support the existence of a transplasma membrane redox system in carrot cells.