SummaryClindamycin (7-chlorolincomycin) was used successfully in the treatment of 3 patients with septicaemia caused by Bacteroides fragilis or related anaerobic Gram-negative bacilli. Bacteroides were isolated from 5 out of 324 blood cultures (1.5%) examined during 1972.Four of the patients were elderly and had undergone major abdominal surgery. It is suggested that, when such patients become pyrexial post-operatively, blood cultures should be taken as a routine procedure and the patients given comprehensive antibiotic treatment immediately. A combination of gentamicin and clindamycin (or parenteral lincomycin) should be satisfactory.