A method of preparing a composition comprising one or more antigens adsorbed to an amino acid wherein said method comprises: (i) mixing a solution of one or more antigens with a solution of the amino acid in an aqueous acid whilst neutralising the mixture of solutions, thereby forming an adsorbate comprising the one or more antigens and the amino acid； (ii) separating the adsorbate into a desired buffer by cross-flow filtration thereby forming said composition； and (iii) recovering said composition； wherein steps (i) to (iii) are performed in a sterile environment and within a closed system.
