The present study was primarily concerned with in vitro investigation of vitamin K2 (MK-4) transport using human placental villous tissues, with the objective of elucidating the placental transport mechanism of vitamin K2. 1. When vitamin K2 (MK-4) was not administered to gravidae, the observed concentrations of vitamin K2 were 0.32 +/- 0.07 ng/ml in the maternal venous blood, undetectable in umbilical venous blood, and 1.01 +/- 0.37 ng/g wet tissue in the placental villous tissue. 2. When vitamin K2 was administered to gravidae, (20 mg/day x 7 days per os), the corresponding concentrations were 0.89 +/- 0.21 ng/ml in the maternal venous blood, 0.18 +/- 0.06 ng/ml in umbilical venous blood, and 5.38 +/- 1.05 ng/g wet tissue in the placental villous tissue. 3. In vitro studies using placental villous tissue indicated that vitamin K2 transport into villous tissue is not active, but suggested the existence of a highly vitamin K2 specific transport system in the human placenta. On the other hand, the results of comparison studies of transport of vitamin K2 and vitamin K1 into villous tissue indicated that transport activity with respect to the former vitamin was higher. Moreover, comparison of vitamin K2 transport into villous tissue at the end of the first trimester and at full term revealed that transport was somewhat higher during the final stage. In summary, the above results indicated that whereas transport of vitamin K1 into the fetus is not especially pronounced, transport into the placental villous tissue is comparatively good.