Abstract: A disposable plastic speculum is used on an infrared sensing thermometer used by placing it in the patient's ear for sensing body temperature through the tympanic membrane. The speculum has the general shape of a funnel with relatively shallow and relatively steep tapered frustoconical sections. The speculum is placed on a speculum retainer on the thermometer for use. The exterior of the speculum retainer has the general shape of interior of the speculum and includes a peripheral enlargement near its smaller end. When the speculum is placed on the retainer it is stretched by the enlargement and thereby retained in place. The speculum is made of a plastic having limited resistance to stress cracking so that the stretched portion over the enlargement cracks during retention on the speculum retainer. Since the speculum thus inherently destroys itself, it is assured that the speculum will be discarded and a new one used for each patient.