Lyme disease (Lyme borreliosis) is a multisystem illness that is caused by the spirochete Borrelia burgdorferi. It is being recognized with increas- ing frequency and in a widening geographic distribution. Patients with Lyme disease typically suffer from an acute flu-like illness associated with a characteristic annular rash (erythema migrans). Most untreated patients go on to develop neurologic, cardiac or rheumatologic disease. The infection is transmitted by the deer tick (Ixodes dammini). In endemic areas, physicians are frequently asked to manage patients who have recently been bitten by a deer tick. The purposes of this study are: 1) to provide precise estimates of the risk of developing subclinical infection or disease caused by B. burgdorferi in patients in an endemic area who report a deer tick bite, and 2) to determine whether antimicrobial (amoxicillin) prophylaxis administered to patients who have been bitten by a deer tick can prevent subclinical infection or disease caused by B. burgdorferi. The study will be a double-blind, placebo-controlled randomized clinical trial in which children and adults who reside in Southeastern Connecticut and have been bitten by a deer tick within the preceding 72 hours will be enrolled. Subjects will be randomly assigned to receive either amoxicillin or a placebo for 10 days. Serum will be obtained from the subjects at the time of enrollment and 5 weeks and 3-4 months after enrollment. Concentra- tions of IgM and IgG antibodies to B. burgdorferi in each sample will be determined by the ELISA method. Subjects will be asked about possible symptoms of Lyme disease 7 days, 3 weeks, 6 weeks, 3-4 months, and 1 year after enrollment. The medical records of the subjects' physicians will also be reviewed for evidence of Lyme disease. The sample size will be sufficient to provide precise estimates (in the placebo group) of the risk of developing subclinical infection or disease caused by B. burgdorferi in patients who are bitten by a deer tick. There will also be sufficient statistical power to determine whether early prophylactic treatment of such patients with amoxicillin can prevent subclinical infection or disease caused by B. burgdorferi in a clinically meaningful proportion of these patients. This study will provide important and clinically useful information about how to manage patients who are bitten by a deer tick, as well as information about the risk of infection that will be useful in planning future studies about the prevention of Lyme disease.