 Subjects are admitted to the NIH Clinical Center for an overnight stay. Two catheters plastic tubes are placed in the subjects arm veins, one for drawing blood samples and the other for injecting one of the following: REG1 drug, REG1 antidote, REG1 drug and antidote, or placebo. Two injections of study medication are given, spaced 3 hours apart, each over a 1-minute period. After each injection, blood is collected at specific times to measure levels of the drug or antidote in the body and the bloods ability to clot. Subjects also provide a 24-hour urine collection and stool sample. 