Gas-liquid chromatographic analysis of volatile fatty acid patterns was performed when growth was initially detected in 87 positive blood cultures. Isovaleric acid, butyric acid, or both were found in 39 of 43 (90%) blood cultures that contained anaerobes but were absent in all 44 cultures that contained facultatively anaerobic or aerobic bacteria. The detection of isovaleric acid indicated the presence of Bacteroides fragilis, as this acid was found in 88% of unimicrobial and 75% of polymicrobial bacteremias due to B. fragilis. Butyric acid was detected most often in blood cultures which grew Fusobacterium spp. (three of three), Clostridium spp. (six of seven), or both (three of four).