Structural transitions in concentrated ovalbumin and bovine serum albumin colloids were studied through rheological and small-angle x-ray scattering (SAXS) measurements. Flow curves and frequency dependence curves of the storage G’ and loss G" moduli change drastically from a plastic-like to Newtonian-like behavior at a concentration range around 40-50 wt %. This is a solid/liquid transition, which is not a transition induced by addition of electrolytes (free ions). These are the first data on this kind of structural transition at high particle concentrations. The SAXS measurements showed that two phases which have different structures coexist in these systems. One phase is an albumin lattice in which the particles (albumin molecules or their associated particles) disperse uniformly with a certain ordered arrangement and the other has a close-packed structure. The structural transition seems to occur when the albumin lattice can no longer be retained because of the increment in the fraction of the phase of the close-packed structure.