We characterized immunoaffinity-purified, undegraded oat (Avena sativa L., cv. Garry) phytochrome by several physicochemical techniques. Phytochrome, of greater than 98% purity [Hunt, R. E., & Pratt, L. H. (1979) Plant Physiol. 64, 332-336], existed in solution as a dimer of its 118 000-dalton