The use of aldehydes other than formaldehyde as fixatives is a comparatively recent development. In 1959 Luft demonstrated that a monoaldehyde, acrolein, could be usefully applied to solve certain problems in the preservation of fine structure. However, it was the extensive studies by Sabatini et al. (1963) that brought the usefulness of aldehydes, especially glutaraldehyde, to the attention of the electron microscopists. These studies indicated that primary fixation with glutaraldehyde followed by secondary fixation with OsO 4 yields satisfactory preservation of fine structure and enzyme activity in a wide variety of specimens. This double fixation has become the standard procedure for preserving both plant and animal specimens including prokaryotes. A mixture of glutaraldehyde and formaldehyde (freshly prepared by depolymerizing paraformaldehyde) has the additional advantage of more rapid penetration into the tissue specimen and is preferred for many specimens. Acrolein in combination with glutaraldehyde has also been employed for certain studies.