The physicochemical properties of Photobacterium lumazine proteins have been investigated. The molecular weights obtained by several physical techniques are in good agreement, and the averages are 2% and 8% higher than the minimum molecular weights from amino acid and ligand content. The average molecular weights, sedimentation coefficients, and molecular radii are respectively the following: Photobacterium leiognathi lumazine protein, 21 200 +/- 300, 2.18 S, and 22.9 A; Photobacterium phosphoreum lumazine protein, 21 300 +/- 500, 2.16 S, and 23.0 A. The hydrations of the lumazine proteins, estimated in several ways, indicate less hydration for P. leiognathi than for P. phosphoreum. The frictional ratios corrected for hydration give axial ratios less than 1.3 for both lumazine proteins. These values agree with those obtained by a combination of rotational and translational frictional parameters and elimination of the common hydrated volume terms. There is insufficient area on the exterior surface to accommodate hydration when the lumzine proteins are considered as smooth-surfaced ellipsoids. The required surface area can be accommodated however by surface roughness with a minimum of 30% internal water.