Electron flow across the plasma membrane of living cells and its rapid modulation by growth factors has been measured continuously through a simple assay procedure whereby the transported electrons are captured by ascorbate free radical to slow the rate of chemical oxidation of ascorbate. The assay provides a direct demonstration of electron transport to an external electron acceptor that is both physiological and impermeant. The reduction of external ascorbate free radical is stimulated by the growth factors, EGF and transferrin, and is inhibited by wheat germ agglutinin. The results demonstrate, under physiological conditions, the operation of a growth factor- and lectin-responsive electron transport system at the cell surface using a cultured human cell line.