Fluorescent x-ray interference patterns have been observed from monolayers of both a metal-containing protein (ferritin) and a nonmetal-containing protein [bovine serum albumin (BSA)] bound on a gold substrate. These interference patterns have been used to determine structure data. The nonmetal-containing protein was first reacted with metal ions by means of a chelate compound to place the necessary chromophore in the molecule. The size of the ferritin core measured by a scanning electron microscope agrees with the value obtained from the x-ray interference experiments. In the BSA molecule, the measured interference fringe is consistent with the model in which the short axes of BSA molecules are perpendicular to the surface substrate.