We have developed a phytochrome immunoaffinity purification procedure that yields undegraded oat (Avena sativa L., cv. Garry) phytochrome of greater than 98% purity within 2 hours when starting with a brushite-purified preparation. Immunoaffinity-purified phytochrome, except for its greater purity, is indistinguishable from conventionally purified phytochrome by gel exclusion chromatography, isoelectric focusing, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We have also used the immunoaffinity technique to purify phytochrome from crude oat extracts, and from brushite-purified pea (Pisum sativum L., cv. Alaska) and rye (Secale cereale L., cv. Balbo) preparations.