When a woman begins to bleed during the first months of pregnancy, great care must be exercised in order to save the pregnancy, particularly if the patient has been suffering from habitual abortion. It is often possible to tide such patients over the critical period after weeks of bed rest combined with hormone and other treatment, thereby saving the lives of children who would otherwise be lost. On the other hand, even when the same precautionary measures and methods of treatment are employed, we frequently encounter failures resulting in abortus after the patients have had to neglect their home responsibilities to no avail for weeks and to undergo a series of costly treatments. We have hitherto had no clinical method for establishing the important differential diagnosis whether pregnancy may or may not be maintained in any particular case. Can this be decided by means of a laboratory test?