Enzyme immunoassay for the in vitro diagnostic quantitative measurement of active free progesterone (a female hormone) in saliva. Measurements obtained by this device may be used in the diagnosis and treatment of disorders of the ovaries or placenta and can be used as an aid for prediction of ovulation. Progesterone (4-pregnene-3, 20-dione) is a C21 steroid hormone containing a keto-group (at C-3) and a double bond between C-4 and C-5. Like other steroids, it is synthesized from cholesterol via a series of enzyme-mediated steps (1) The steroid hormone Progesterone is a female sex hormone which, in conjunction with estrogens, regulates the accessory organs during the menstrual cycle and it is particularly important in preparing the endometrium for the implantation of the blastocyte and in maintaining pregnancy (2) In non-pregnant women progesterone is mainly secreted by the corpus luteum whereas in pregnancy the placenta becomes the major source (3,4). Minor sources for progesterone are the adrenal cortex for both sexes and the testes for males. The Progesterone level in saliva represents the concentration of the active free Progesterone. The DRG Salivary Progesterone ELISA kit is based on the competition principle and the microplate separation. An unknown amount of Progesterone present in the sample and a fixed amount of Progesterone conjugated with horseradish peroxidase compete for the binding sites of a rabbit polyclonal Progesterone -antiserum coated onto the wells. After one hour incubation the microplate is washed to stop the competition reaction. After adding the substrate solution, the concentration of Progesterone is inversely proportional to the optical density measured.