Density gradient centrifugation was used for identification of particulates in vaccines formulated with aluminum phosphate (AlPO(4)) adjuvant. Protein-derived particulates are sporadically seen in protein pre-filled formulations where siliconized syringes are employed. These particulates are difficult to distinguish from the AlPO(4) suspension. Prior to centrifugation, Histopaque solution is placed at the bottom of the test tube (or the original syringe), resulting in a layer beneath the suspension. After brief centrifugation, AlPO(4) settles to the bottom of the test tube, while the light-weighted particulates in the sample become visible, floating in the inter-phase between the two layers.