(T-4)-Osmium oxide (OsO4). A highly toxic and volatile oxide of osmium used in industry as an oxidizing agent. It is also used as a histological fixative and stain and as a synovectomy agent in arthritic joints. Its vapor can cause eye, skin, and lung damage.

A general purpose reagent is a chemical reagent that has general laboratory application, that is used to collect, prepare, and examine specimens from the human body for diagnostic histopathology, cytology, and hematology, and that is not labeled or otherwise intended for a specific diagnostic application. General purpose reagents include cytological preservatives, decalcifying reagents, fixatives and adhesives, tissue processing reagents, isotonic solutions, and pH buffers.

An osmium oxide composed of four oxygen atoms bound to a central osmium atom. Osmium tetroxide is most commonly used as a fixative, and in oxidation reactions and biological staining. It is characterized as a colorless, or pale-yellow crystalline solid with a pungent odor. Exposure occurs by inhalation, ingestion, or contact.

The chemical agent osmium tetroxide.