The concentrations of ciprofloxacin produced in bone, cartilage and menisci after a single administration of 200 mg were determined at different intervals in a group of patients with an average age of 80 years. Concentrations of 0.11 to 0.94 mg/kg bone tissue were measured after 0.5 to 5 hours. In the cartilage a concentration of active substance was measureable only once (4.18 mg/kg). In the presence of marked circulatory disorders the active substance concentrations reached in the bone were above those found in the seriously damaged muscle. Although the concentrations reached in the bone are effective, no risk should be taken in osteomyelitis. Ciprofloxacin should therefore be used at high dosage and possibly be combined with another substance. Given for therapeutic purposes, a single dose of ciprofloxacin is naturally not effective enough, and given for prophylactic purposes, not safe enough to prevent a post-traumatic osteitis.