Amniotic fluid contains diamine oxidase (DAO) in high concentration, almost equal to that in maternal blood plasma (Tornqvist et. al, 1971). After rupture of the membranes the enzyme is demonstrable in the vagina and in the absence of vaginal bleeding the measurement of the DAO content of vaginal fluid has been shown to be a reliable method of resolving doubt about whether or not the membranes have ruptured.