A simple procedure for determination of vitamin K-1 was developed for routine compliance monitoring of supplemented infant formula and measurement of endogenous levels in milk and milk powders. Samples are digested with lipase and extracted into hexane; an aliquot is evaporated, reconstituted in methanol, and analyzed by reversed-phase LC. Post-column zinc reduction of phylloquinone facilitates detection by fluorescence. The procedure was subjected to an AOAC collaborative study involving 8 materials, each in blind duplicate, across the range of 5-120 mu g/100 g solids and including NIST 1846 reference material. Thirty-three laboratories returned valid data which were then statistically analyzed for outliers and precision parameters. Mean RSDR (%) was 6.53 (4.33-10.94), with a mean HORRAT value of 0.33 (0.23-0.43) and RSDr:RSDR ratio of 0.74. K-1 isomers (cis and trans) were aggregated with conventional C-18 columns, but may be selectively estimated with use of the C-30 column.