The present invention relates to a method for sterilisation of phospholipid suspensions, useful in the preparation of ultrasound contrast agent precursors comprising phospholipid-stabilised perfluorobutane microbubbles. The method provides sterility assurance, without undue thermal degradation of the phospholipid. The method is also amenable to commercial scale manufacture. Also provided are methods of preparing kits and ultrasound contrast agents incorporating the sterilisation method of the invention.