Beryllium is ideal for coating biological specimens for light element X-ray microanalysis at low temperature. It has higher electrical and thermal conductivity at 100 K and lower absorption of X-rays of biological interest than carbon, aluminium or chromium. It produces no detectable characteristic X-rays. When adequate precautions are taken beryllium is a valuable alternative to other coating materials. Because of its toxicity, however, it should not be used indiscriminately.