During the period 1980-1987, Clostridium septicum was identified at Statens Seruminstitut, from anaerobic blood cultures in six patients. Five of the patients had cancer of the colon or rectum (c. coli) and one had cancer of the ovary. Correspondingly great incidence of solid tumours, particularly c. coli with C. septicum infection without preceding trauma or compromizing of immunity has been reported in the literature. Meticulous bacteriological identification should be undertaken on anaerobic culture as the finding of C. septicum in the blood should instigate investigation for c. coli or another form of solid tumour if no other explanation for the infection is available.