THERE has been considerable speculation concerning the role of vitamin K in photosynthesis. Arnon1 has shown that several water-soluble naphthoquinones stimulate photosynthetic phosphorylation, although vitamin K1 is inactive as a stimulant in these reactions. Dam2 has shown that material which shows the biological activity of vitamin K is contained in chloroplasts and is localized in the green parts of plants. On the other hand, vitamin K1 has not been isolated from chloroplasts and has not been detected previously by chemical assay in extracts from chloroplasts3. In our laboratory we have not previously been able to observe vitamin K1 in extracts from chloroplasts even though we could find the vitamin in extracts from whole leaves. We have now found chemical evidence for vitamin K1 in extracts from spinach chloroplasts. The compound has been purified by silicic acid column chromatography and has shown absorption maxima at 269, 261, 249, 242 mµ in alcoholic solution. After treatment with potassium borohydride the absorption decreases in the 260–270 mµ region and increases to a new maximum at 244 mµ with an isosbestic point at 254 mµ. This material also gives a positive Irrevere–Sullivan test4 as well as a positive Dam–Karar test5 for vitamin K.