IT was generally accepted that vampire bats were bloodsucking animals until 1932 when Dunn1 observed that these animals use their tongues to lap blood from freely bleeding wounds inflicted by means of razor-sharp superior incisors. This observation was confirmed by Ditmars and Greenhall2 and by many subsequent investigators, and it has been noticed that bites inflicted by vampire bats not only bleed very freely when the wound is first made, but also continue to bleed for several hours after the bat has ceased to feed3. It is therefore of interest to investigate the mechanism by which vampire bat saliva brings about this degree of interference with the haemostatic mechanism of the host animal.