Osmium tetroxide fixation of tissue blocks, as usually effected, is preceded by an acidification of the tissue. This acidification is probably responsible for morphological alterations which are notably disturbing in electron microscopy. The acidification and the resulting morphological alterations cannot be prevented by homogenizing the tissue directly in OsO(4) solutions or by adding enzyme inhibitors (fluoride, iodoscetamide) to the fixative. Fixation experiments with buffered OsO(4) solutions have shown that the appearance of the fixed cells is conditioned by the pH of the fixative. The quality of fixation can be materially improved by buffering the OsO(4) solutions at pH 7.3-7.5, The acetate-veronal buffer appeared to be the most favorable of the buffers tested, Because of these findings, 1 per cent OsO(4) buffered at pH 7.3-7.5 with acetate-veronal buffer is recommended as an appropriate fixative for electron microscopy.