It was found that the photoexitation of a frozen solution of vitamin K1(phyllonaphtoquinone) gave rise to the formation of biradicals with a very bright spectra (ΔH ∼ 200 G), typical for randomly oriented radical pairs with a summary spin of S = 1. This spectra was not overlaped by those of the other radical products generated following the photolysis of vitamin K1in different matrices. This method is proposed for the determination of vitamin K, in biological samples and was successful in following the accumulation of vitamin K1in mouse liver following subcutaneous injection.