‘Septic shock ’ is produced most commonly by Gram-negative bacilli derived from pre-existing infections in various parts of the body. Anaerobic bacteraemic states are most frequently due to Bacteroides fragilis and are usually derived from sepsis in the gastrointestinal or female genital tracts, or from aspiration pneumonia. Among aerobic bacilli, Pseudomonas aeruginosa has emerged as an increasingly frequent cause of septic shock, especially in leukaemic patients. For the treatment of septic shock of unknown cause gentamicin is the drug of first choice; if anaerobes are thought to be implicated metronidazole should be added. The possible presence of Ps. aeruginosa or streptococci requires the addition of carbenicillin or benzylpenicillin respectively.