Bouin-Allen fixing solution is a fixing solution mainly used in botany, especially for plant tissue fixation. This solution enables the preservation of fine cellular structures and delicate plant materials, and is therefore preferred when detailed studies of plant structures are required.

Bouin-Allen Fixing Solution consists of a mixture of picric acid, formalin, acetic acid and chromium(VI) oxide. The chromium(VI) oxide in the solution enhances the fixation properties and contributes to better preservation of cellular structures.
In pathology, the use of this fixative solution is less common because there are alternative fixative solutions that are better suited for the study of animal tissues. However, it can be used in special cases when fixation of plant tissue is required in pathology, for example, in the study of plant pathogens or plant-animal interactions.

The post-treatment is relatively laborious, but it results in high-contrast and brilliant staining of histological sections.