A review of the types of organisms and their sensitivities to antibiotics, as well as the source of infection, was carried out for 11 patients who had late onset infective endocarditis and a prosthetic heart valve. Candida organisms were isolated from two patients. In nine patients with late onset bacterial endocarditis, the organisms isolated were five streptococci and four staphylococci. Their sensitivities to antibiotics were penicillin, five of eight; erythromycin, eight of eight; and cephalothin, six of seven. Two patients with endocarditis had ulcerations beneath their dentures, and one had undergond a prior dental procedure. Of 52 healthy patients with prosthetic valves who were interviewed, only 18 had visited a dentist during the previous year, and six did not receive antibiotic prophylaxis for endocarditis. It is concluded that patients with prosthetic heart valves do not practice good oral hygiene and, if they do visit the dentist, some may not receive antibiotics for endocarditis prevention. An antibiotic regimen for endocarditis is presented that is consistent with the organisms found in the oral cavity and those found in patients with endocarditis who have a prosthetic valve.