An ultrathin frozen section is suitable for demonstrating cytochrome c oxidase activity at electron microscopic level.

Cytochrome c oxidase activity in rat liver was demonstrated in 40 microns sections and ultrathin frozen sections (100-200 nm in thickness), using the diaminobenzidine method. Electron microscopic observations showed strong activity in 81.9 +/- 13.1% of mitochondria along the edge, and 29.9 +/- 21.2% in the center of 40 microns sections. On the other hand, all mitochondria possessed strong activity in ultrathin frozen sections under the same experimental condition. In the ultrathin frozen section, the mitochondrial membranes, as well as the plasma membrane, were broken. The present study indicates that the ultrathin frozen section is suitable for demonstrating cytochrome c oxidase activity at electron microscopic level.