A method includes (i) adding allantoin in a supersaturating amount to a protein preparation including a desired protein and at least one endotoxin as a contaminant, (ii) removing solids after the adding step to provide a sample for further purification by void exclusion chromatography on a packed particle bed of electropositive particles in a column, the packed particle bed having an interparticle volume, (iii) applying a sample volume to the packed particle bed, wherein the electropositive particles support void exclusion chromatography, and wherein the sample volume is not greater than the interparticle volume, and (iv) eluting a purified sample including the desired protein and a reduced amount of the endotoxin. The method is optionally carried out with only the allantoin treatment or only the void exclusion chromatography.