Late infection of a total hip arthroplasty after dental extraction has been reported, but never with an organism that is found exclusively in mouth flora. Actinomyces israelii is an organism responsible for dental caries. A 61-year-old woman developed an infected total hip arthroplasty after dental work. She denies ever being instructed to take prophylactic antibiotics by her orthopedic surgeon, by her internist, or by her dentist. Considering the extensive morbidity and potential mortality of an infected hip prosthesis, it is essential that all physicians are aware of the indications for antibiotic prophylaxis following joint arthroplasty. Recommendations for antibiotic prophylaxis for dental manipulation are a loading dose of 2.0 g of penicillin V orally or 1.2 million U of aqueous procaine penicillin G with 1.0 gram of streptomycin given intramuscularly 30 minutes before dental work, followed by four doses of 0.5 g of penicillin V orally every six hours.