After optimizing conditions for maximal production of menaquinones (MK), S. aureus and B. vulgatus were grown in batches, harvested and extracted for qualitative and quantitative MK content utilizing HPLC (high performance liquid chromatography) until a total of 6 mg was available. Five normal healthy male volunteers were placed on a vitamin K1 deficient diet (< or = 25 micrograms/day) and were subsequently warfarinized to maintain a prothrombin time (PT) 1.5-2 times control. Following stabilization of daily warfarin dosage 1 mg doses of the extracted MK were orally administered. As a control, the same volunteers were later warfarinized but no MK was given. Within 24 h of MK administration the prothrombin time (PT) decreased (mean +/- SEM) 3.6 +/- 1.0 s (p < 0.005) and the Factor VII level increased 0.36 +/- 0.3 u/ml (p < 0.005) vs a PT increase of 1.0 +/- 1.0 s (p > 0.1) and a Factor VII level increase of 0.03 +/- 0.1 u/ml (p > 0.1) in the control phase. Within 48 h of MK administration the PT was normal in all subjects but remained > or = 1.5 times control in the control phase. These data demonstrate for the first time the absorption and bioactivity of bacterially synthesized vitamin K in humans.