S-100, a protein originally believed to be unique to the nervous system, has recently been found in extraneural presence of S-100 in the testis, namely in Leydig cells and in lymphatic endothelial cells, using immunohistochemical and immunochemical methods. We show that the protein in the testis is immunologically identical to brain S-100. The S-100-labelled cells in the testis exhibit morphological similarities with other cell types in different tissues known to contain S-100.