Ninety intestinal organisms (71 isolates from fecal samples of neutropenic patients with cancer or from various sites in patients with intraabdominal infections and 19 control strains) were examined by reverse-phase thin-layer chromatography for their ability to produce menaquinones in vitro. Menaquinones were found in all of 24 organisms of the Bacteroides fragilis group. Two other Bacteroides species, Bacteroides disiens and Bacteroides bivius, also produced menaquinones. A single isolate of Bacteroides species lacked menaquinones. These constituents were found in all of five strains of Escherichia coli, all of four strains of Klebsiella pneumoniae, five of eight strains of Propionibacterium species, two of five strains of Eubacterium species, and the one strain each of Arachnia propionica and Veillonella parvula tested. No menaquinones were detected in organisms of the genera Fusobacterium, Clostridium, Bifidobacterium, Lactobacillus, Actinomyces, Peptococcus, or Peptostreptococcus. These findings suggest that E. coli, Bacteroides species, and some gram-positive, anaerobic, non-spore-forming bacilli produce menaquinones that may be a source of vitamin K in patients who are deprived of exogenous vitamin K1.