When ferritin is reconstituted from Fe and apoferritin in vitro in the presence of Pi, the product obtained differs both from native ferritin and from ferritin reconstituted in the absence of Pi. When the latter is incubated with Pi the product resembles native ferritin with respect both to the pattern of Pi incorporated per molecule or per Fe atom and to the ease of release of this Pi relative to Fe release. It is concluded that much of the Pi of native ferritin is adsorbed on surfaces of ferritin iron-core crystallites. The results also suggest that Pi is not present at the intracellular site of Fe incorporation into ferritin, but is added after Fe.