Bouin solution, or Bouin's solution, is a compound fixative used in histology. It was invented by French biologist Pol Bouin and is composed of picric acid, acetic acid and formaldehyde in an aqueous solution. Bouin's fluid is especially useful for fixation of gastrointestinal tract biopsies because this fixative allows crisper and better nuclear staining than 10% neutral-buffered formalin. It is a good fixative when tissue structure with a soft and delicate texture must be preserved.