596671 ASSAYS FOR INFLUENZA VIRUS HEMAGGLUTININS Provided is a method for purifying influenza virus HA in a sample, comprising steps of: (i) ultrafiltration of the sample using a membrane which retains any denatured or aggregated HA to provide a filtrate and (ii) RP-HPLC of the filtrate to separate any HA therein from any other components therein. Further provided are specific membrane cut-offs and RP-HPLC support materials for the method.