The complex ESR signal previously observed in fatty tissues has been identified. It is a copper low-molecular-weight sulphur compound complex similar to copper-dithiocarbamate. The usual source of this signal is contact of copper-containing tissue with surgeon's gloves. This result emphasizes the care that must be taken in obtaining and interpreting ESR spectra of tissues. The formation of this complex by intentionally adding dithiocarbamate to tissue can be used as a very simple and sensitive analytical method for tissue copper. Copper levels of less than 0-1 ppm can be detected by this method. This is an order of magnitude more sensitive than determinations by atomic absorption.