Adjuvant System 04 or AS04 is a trade name for a combination of adjuvants used in various vaccine products by GlaxoSmithKline, in particular the Fendrix hepatitis B vaccine and Cervarix human papillomavirus (HPV) vaccine. It consists of aluminium hydroxide and monophosphoryl lipid A (MPL). It is the successor of AS03, a squalene based adjuvant used in H1N1/09 and H5N1 influenza vaccines.