Intravascular or cardiac endothelial lesions may become colonized during bacteraemic episodes and lead to the development of bacterial endocarditis (BE). It has therefore long been recommended that patients with known cardiac lesions receive prophylactic antibiotics before undergoing procedures that might release bacteria into the blood stream. Because clinical trials of antibiotic prophylaxis of endocarditis cannot be conducted in humans for ethical as well as for statistical reasons (Durack, 1985), the questions of which antibiotic, what dosage, and for how long are a matter of controversy. Unfortunately, these questions can only be studied in animals, with all the limitations that this type of approach brings with it. However, animal experimental studies have helped in understanding the conditions and, to some extent, the mode of action of antibiotics in preventing the development of endocardial infection, thus allowing some rationale for devising prophylactic recommendations for the various patients at risk of developing BE.