The solubilization of vitamin K1 by bile salts (sodium deoxycholate, sodium cholate and their corresponding glycine conjugates) and phosphatidylcholine (egg)-bile salt mixed micelles has been investigated. The solubilization curves were not always linear with increasing bile salts, but the vitamin was appreciably solubilized in the region below their CMCs. In the bile salt solutions (20 mM, phosphate buffered saline, pH 7.5, ions strength 0.2), the solubilized vitamin ranged from 0.3 to 0.9 mM. With increasing phosphatidylcholine, the amount of vitamin solubilized was dramatically increased; at the molar ratio of 1:1 (both 20 mM), the amount of vitamin solubilized was about 25-30 times more than by the corresponding bile salts alone. There is a possibility that exogenous phospholipid given orally as liposomal forms assists the solubilization of vitamin K1, in the intestine. This characteristic is suggested as being responsible, in part, for the enhanced recovery of blood coagulation after oral administration of liposomal vitamin K1 to warfarin-treated rabbits.