A process for purifying lysine in a lysine production process, which involves passing a solution containing lysine over a cation exchange resin such that lysine adsorbs onto the resin, eluting adsorbed lysine from the resin with an aqueous solution of ammonia to produce a lysine-containing elution liquid, passing the elution liquid directly to a reverse osmosis membrane system, and concentrating the lysine-containing elution liquid with the reverse osmosis membrane system such that ammonia is selectively removed from the lysine containing elution; the aqueous ammonia which is separated from the lysine may be recycled through the cation exchange resin to regenerate the resin or to elute more lysine from the resin.