Vitamin P was postulated by Szent-Györgyi1as a regulator of vascular permeability, isolated in crystalline form as a flavone glucoside from lemon juice and named citrin to indicate its chemistry. It was found to consist of mixed crystals of two related flavone dyes, the glucoside hesperidin C50H60O27and the glucoside of eriodictyol; the former constitutes the major part of citrin but the latter is responsible for the chemical reactivity and color production. The glucosides of citrin are but two forms of the same flavonone glucoside, the eriodictyol glucoside being formed from hesperidin by demethylation during ripening of citrus fruits, particularly lemon, orange and grapefruit. Citrin forms light yellow crystals sparingly soluble in water but very soluble in alkali, giving intense yellow solutions. The relative composition of the solid and liquid phases, respectively, is determined by the solubility of its two components; the crystals consist