Ranchers have observed that residual hemorrhage decreases after livestock suffer repeated bites from common vampire bats (Desmodus rotundus). We hypothesized that repeated exposure to the anticoagulants in the bats' saliva generated an immune response. We compared clotting time of blood mixed with Desmodus saliva between individuals of several species of livestock that were regularly exposed to vampire bats (“bat-attacked”) and individuals living where bat predation was not observed (“bat-naïve”). We also compared the bleeding times of domestic sheep following a single defensive bite from a bat, before and after subjecting the animals to a series of feeding bites by Desmodus. Clotting time was significantly shorter in bat-attacked than in bat-naïve livestock. Bleeding time after a provoked defensive bat bite was significantly shorter after sheep were exposed to a series of feeding bites from Desmodus. Bat predation induced increased resistance by the prey against the anticoagulants, suggesting an immune response.