The in situ hybridization (ISH) method is generally used in light microscopy but not in electron microscopy. We investigated whether the ISH method can be applied to electron microscopy (EM) by a preembedding method using human lymphocyte nuclei as a material. In addition, we also present some data from fluorescence in situ hybridization (FISH) using the same DNA probe. We indicate the suitability of preembedding EM-ISH to isolated cells.