THE rate of synthesis of hæmoproteins has been studied by measuring their incorporation of iron-59. Most of these studies have concerned hæmoglobin, but some1,2 also myoglobin, catalase, cytochrome c and cytochrome b5. No report on the incorporation of iron-59 into cytochrome oxidase has yet appeared. Except for cytochrome c and cytochrome oxidase, the above pigments all contain protohæm, so that measurement of their individual uptake of iron-59 requires their previous separation, which is often difficult.