TETRAZOLIUM salts, first prepared by Pechmann and Runge1, have attracted the interest of workers in biological fields since the demonstration by Kuhn and Jerchel2 that living organisms such as yeast, garden cress and bacteria have the power of reducing the colourless, soluble tetrazolium salts to red, insoluble formazan compounds, reduction taking place in neutral solution and the cells taking up the colour. Mattson et al.3 confirmed these results and showed that a similar reaction was given by many other viable tissues of both plant and animal origin ; they suggested that reduction is due to dehydrogenase systems requiring co-enzymes I or II. Straus et al.4 applied the test to normal and neoplastic human tissues. They argued that, as malignant tumours show an increased glycolytic activity, their reducing power for tetrazolium salts might also be augmented. These authors claim that this hypothesis is confirmed by their initial results.