The use of Toxavit for pulpal devitalisation in a 30-year-old woman is presented. Improper use of this material resulted in necrosis of soft tissues and bone. The bone sequestrum was removed and examined under an electron microscope, revealing thick accumulation of plaque on the surface of the bone sequestrum and dissolution of bone material under the influence of microorgansims. Keywords: bone sequestrum, devitalisation, paraformaldehyde