Abstract 1. 1.|Glycoprotein I was shown to be free from major contaminants by the results of immunoelectrophoresis and immunodiffusion and electrofocusing experiments. A minor and occasional contaminant of higher iso-electric point which was active in hydrolysing α-N- benzoyl- l -arginine p-nitroanilide and amounted to about 0.1% of the total protein could however be demonstrated in the leading part of DEAE-cellulose chromatograms developed with suboptimum concentrations of NaCl. Glycoprotein I freed from this contaminant was shown to undergo autodigestion and, furthermore, to have attained a constant specific activity as measured by the number of amino groups liberated or by the extent of a spectral blue-shift. 2. 2.|Glycoprotein I was also shown to be homogeneous by velocity and high-speed equilibrium centrifugation. Its molecular weight was 60 000 ± 1000. Its hydrodynamic properties ([η] = 0.053 ± 0.003 dl/g; Ks/[η] – 1.64; β = 2.67 · 10−6; and Ve = 1.18 · 10−18 ml) taken in conjunction with previous titration data indicated a compact protein core with highly hydrated and loose surface carbohydrate structures. It is suggested that the physiological functioning of seed glycoproteins may be dependent on this type of molecular arrangement.