Little data are available with regard to the amount and types of bacterially synthesized menaquinones within the human intestinal tract. Quantitative reverse phase high performance liquid chromatographic (HPLC) analysis of the distal colonic contents of 10 male volunteers revealed the following: (mean +/- SEM) micrograms/g dry weight: MK-4 to MK-7 5.55 (+/- 0.27); MK-9,10 14.54 +/- 0.29; K1 1.31 (+/- 0.14). Similar analyses of the colostomy contents of subjects revealed a total menaquinone content of 8.85 micrograms/g dry weight. The total intestinal contents obtained from five subjects after isotonic saline purgation qualitatively revealed MK-4 to MK-11 with a mean (+/- SEM) content of MK-4 to MK-10 of 5.4 +/- 2.1 micrograms/g dry weight or a mean (+/- SEM) of 1.6 +/- 0.7 mg dry weight of the total contents (243 g). The quantitative mean growth for the major menaquinone producers Escherichia coli and Bacteroides species was 5.2 +/- 3.2 and 9.2 +/- 1.8 log10 CFU/g dry weight, respectively. These results indicate that menaquinones are present in significant concentration in the human intestinal tract, and a potentially large reservoir of vitamin K may be available to the host.