Participants will have 40 to 60 cc (1 to 2 ounces) of blood drawn at seven intervals. The first collection will be as soon as possible after exposure to hepatitis C virus and then again at 2, 4, 6, 12, 24, and 48 weeks after exposure. The white blood cells will be studied for their response to the virus, and markers for infection will be followed. If infection develops, additional samples of blood may be requested, and patients will be offered evaluation for treatment. Test results will be kept confidential and will not be entered into any medical records.