The complement of soluble proteins contained in the five annual leaf sets and the apical region of the shoot of cinnamon fern, Osmunda cinnamomea L., has been determined using disc electrophoresis. The apical region and the very young leaf primordia contained high and comparable numbers of soluble proteins. Older primordia and mature leaves contained progressively fewer soluble proteins, except that the number of proteins increased in the oldest sets of leaves. These findings are discussed in relation to the results reported for the leaves of some flowering plants.