The efficacy and safety of ciprofloxacin in the treatment of 68 episodes of bacteremia were studied. Patients were treated intravenously (30 cases), orally (13 cases), or with sequential intravenous/oral therapy (25 cases). Intravenous doses ranged from 200 to 400 mg per day and oral doses ranged from 1,000 to 1,500 mg per day. According to the criteria of McCabe and Jackson, 39 cases had nonfatal and 29 had ultimately fatal underlying diseases. The clinical condition of patients at the start of therapy was critical or poor in 40 cases and fair or good in 28. Sixty-four of the 68 episodes of bacteremia were monomicrobial and the remaining four were polymicrobial. The causative micro-organisms were: Escherichia coli (18 episodes), Pseudomonas aeruginosa (13 episodes), Acinetobacter sp. (10 episodes), Salmonella sp. (seven episodes), Enterobacter sp. (six episodes), Proteus sp. (four episodes), Serratia sp. (four episodes), Haemophilus influenzae (three episodes), Klebsiella sp. (three episodes), Staphylococcus aureus (2 episodes), and Morganella morganii (two episodes). Overall clinical efficacy of ciprofloxacin was 94 percent (64 of 68 patients). Bacteremia persisted in four patients (failure rate of 6 percent). Five organisms persisted: Acinetobacter sp. (two patients), P. aeruginosa (one patient), Enterobacter sp. (one patient), and Serratia sp. (one patient). Side effects were phlebitis associated with intravenous administration (four cases), dizziness (four cases), and superinfection (six cases). Superinfecting organisms and sites were as follows: Enterococcus faecalis, wound (2 cases); Candida sp., urinary tract infection (one case); Acinetobacter anitratus (ciprofloxacin resistant), urinary tract infection (one case); Staphylococcus epidermidis, blood (one case); and Clostridium perfringens, blood (one case). Ciprofloxacin administered either intravenously, orally, or intravenously followed by the oral route is effective therapy in the treatment of severe bacteremic infections.