Synthesis of nanometer-size magnetic particles within the confined environment of apoferritin might have several uses in biomedical applications and in magnetic information storage devices. Successful use of these protein-encapsulated mineral particles in industrial and biomedical applications depends on the structural stability of these particles at various interfaces. We studied adsorption of apoferritin and magnetoferritin at the air-water interface. Transmission electron microscopy analyses of the protein films at the air-water interface indicate that the spherical protein shell of both apoferritin and magnetoferritin collapses into a two-dimensional disk shape at the interface. In the collapsed state, the magnetite core is released from the protein shell. The results suggest that the oligomeric state of magnetoferritin is not stable at hydrophobic surfaces.