Carbon nanotubes, compounded into plastic create bulk electrodes, form the basis of a novel sensor for biological assays. The nanotubes function as both a solid phase and working electrode for generation of electrochemiluminescence, the detection method used in this system. The Figure shows a scanning electron microscopy image of a composite used for an immunoassay. The bright spots in the image are 10 nm diameter gold colloids covalently attached to antibodies. The inset shows a region of a composite with no gold colloids.