It is unclear whether menaquinones produced by the intestinal microflora play any role in human nutrition. Reports of coagulopathy due to vitamin K deficiency occurring in patients receiving broad spectrum antibiotics indirectly suggest that vitamin K2 produced by the gut microflora may be utilized by the host. We analyzed the vitamin K1 (phylloquinone) and vitamin K2 (menaquinone) content in a convenience sample of 22 human post-mortem liver samples, including 9 individuals who had been receiving broad spectrum antimicrobials prior to death and 13 individuals who had been victims of sudden, unexpected deaths. There were no significant differences in the mean (+/- SEM) phylloquinone content between the 2 groups [21.9 (+/- 15.5) vs. 16.0 (+/- 9.3) pmol/g wet weight (excluding those who had received supplemental vitamin K1)] but there was a significant difference (p < 0.05) in the total menaquinone (MK) content, 70.0 (+/- 23.3) vs. 423.1 (+/- 141) pmol/g between the 2 groups. These findings suggest an association between receipt of broad spectrum antibiotics and a reduction in hepatic menaquinone concentration, lending support to the hypothesis that a reduction in the gut microflora responsible for their production leads to reduced hepatic stores of this form of the vitamin.