Triple Sugar Iron agar failed to detect hydrogen sulphide in 44 out of 69 hydrogen-sulphide producing strains of Enterobacteriaceae, which at the same time fermented lactose and/or sucrose. The species involved were Salmonella typhi, Salmonella enteritidis, Citrobacter freundii, Escherichia coli, and Proteus vulgaris. By contrast, no false-negative reactions were observed in 74 strains, which fermented neither lactose nor sucrose. Failure to detect hydrogen sulphide was probably due to acidification of the medium following the fermentation of carbohydrates. A medium without carbohydrates is preferable in diagnostic situations where hydrogen-sulphide detection is of great importance.