1. The first stage in the preparation of a specimen for microscopical examination, in which the tissue is killed and preserved in as natural a state as possible by immersion in a chemical fixative. The fixative prevents the distortion of cell components by denaturing its constituent protein. Some commonly used fixatives are formaldehyde, ethanol, and Bouin’s fluid (for light microscopy), and osmium tetroxide and glutaraldehyde (for electron microscopy). Fixation may also be brought about by heat.

2. See nitrogen fixation.