Zenker's fixative is a rapid-acting fixative for animal tissues. It is employed to prepare specimens of animal or vegetable tissues for microscopic study. It provides excellent fixation of nuclear chromatin, connective tissue fibers and some cytoplasmic features, but does not preserve delicate cytoplasmic organelles such as mitochondria. Helly's fixative is preferable for traditional dye staining of mitochondria. Zenker's fixative permeabilises the plasma, but not the nuclear membrane. It can therefore be used to selectively stain mitotic cells (where the nuclear membrane has dissolved) with antibodies against chromatin

Zenker's fixative contains mercuric chloride ("corrosive sublimate"), potassium dichromate, sodium sulfate, water, and acetic acid. Fixatives containing mercuric chloride or potassium dichromate are toxic, making disposal as hazardous waste costly. Mercuric chloride can be replaced with the same weight of less toxic zinc chloride, but the resulting "zinc-Zenker" may not give the same quality of fixation as the original mixture.

This fixative is named after Konrad Zenker, a German histologist, who died in 1894 (Baker 1958).