Using a sensitive electrochemical assay for vitamin K1 and standardized techniques for breast-milk collection, we studied the vitamin K1 content of human milk during the first 5 wk of lactation with respect to 1) individual and interindividual differences, 2) the relationship of vitamin K1 to other lipids, and 3) the influence of oral supplements of vitamin K1 on breast milk concentrations. Comparison of fore and hind milk from the mothers revealed higher vitamin K1 concentrations in hindmilks, suggesting that the lipid content influences the vitamin K1 concentration in maternal milk. Samples of maternal milk from nine mothers collected from day 1 to day 36 of lactation showed significantly higher vitamin K1 concentrations in colostral milk than in mature milk. For colostral milk there was a significant correlation of vitamin K1 to cholesterol (r = 0.62) but not to total lipid or phospholipid suggesting a role for cholesterol in the secretion of vitamin K1 into colostral milk. For mature milk correlation coefficients of vitamin K1 with all lipids were low (r = 0.29-0.37) suggesting that at later stages of lactation dietary fluctuations of vitamin K1 may be a more important determinant of the vitamin K1 content of breast milk than the lipid composition. To test the influence of diet, mothers were given oral supplements of vitamin K1.(ABSTRACT TRUNCATED AT 250 WORDS)