The high capacity of proliferating mammalian cells to transfer electrons from cytosolic NADH to extracellular electron acceptors like oxygen is poorly understood and not widely recognized. Nevertheless, trans-plasma membrane electron transport (plasma membrane redox control) probably ranks alongside the Na+/H+ antiport system (pH control) and glucose transport in facilitating cellular responses to physiological stimuli. These plasma membrane transport systems are acutely responsive to receptor ligation by growth factors, polypeptide hormones, and other cell activators. A novel tetrazolium-based cell proliferation assay that we have shown to measure an NADH-oxidoreductase component of the trans-plasma membrane electron transport system has allowed direct comparisons with NADH:ferricyanide-oxidoreductase and respiratory burst NADPH-oxidoreductase. In addition, an NAD(P)H-oxidase at the cell surface and an NADH-oxidase activity in body fluids can be measured by modifying the basic cell proliferation assay. As determined by reduction of the cell-impermeable tetrazolium reagent, WST-1, electron transfer across the plasma membrane of dividing cells can exceed that of fully activated human peripheral blood neutrophils. Cellular reduction of WST-1 is dependent on the presence of an intermediate electron acceptor and is inhibited by superoxide dismutase (SOD) and by oxygen, implying indirect involvement of superoxide in WST-1 reduction. Cell-surface NAD(P)H-oxidase and serum NADH-oxidase are shown to be distinct from trans-plasma membrane NADH-oxidoreductase by their differential sensitivity to capsaicin and pCMBS. The glycolytic metabolism of cancer cells may be linked to changes in trans-plasma membrane NADH:WST-1-oxidoreductase activity and to increased serum NADH-oxidase in cancer.