By quantitative gas-liquid chromatography of glucose-containing blood cultures at the moment of first signs of growth, a presumptive diagnosis of anaerobic bacteremia could be made in 24 out of 26 cultures yielding obligate anaerobes upon subsequent culture. With Bacteroides sp. (20 strains isolated), elevated levels of isovaleric acid (greater than or equal to 0.1 mumol/ml) and/or succinic acid (greater than or equal to 5 mumol/ml) were detected in the medium used. An exception to this were two strains of Bacteroides thetaiotaomicron that did not produce sufficient quantities of these acids. In the case of butyrate-producing gram-positive cocci (four strains), butyric acid in amounts of greater than or equal to 0.8 mumol/ml was detected. Propionibacteria (five strains) produced propionic acid in amounts of greater than or equal to 3.9 mumol/ml. No false positive results were found in 103 blood cultures with growth of aerobic or facultatively anaerobic bacteria only.