Plant ferritin is generally assumed to release its iron in green or greening plastids. We studied ferritin iron mobilization under ”illuminated stroma” conditions: 50mM ascorbate, pH 8.0 and oxygen. Under those conditions ferritin releases its iron, after a lag of about 4 hours, at a rate of 8 % per hour at25°C. 10'5 MCu+ + decreases the lag to 1 hour, and increases the rate to 20% per hour; oxygen is necessary for the reaction, and catalase inhibits strongly. We conclude that an oxidation product of ascorbate, or an intermediate produced by its interaction with ferritin and oxygen, reduces ferritin iron. This compound is unstable or has a high affinity for ferritin iron.