The effect of adjusting the pH of a model ovalbumin (iep = 4.7) and aluminum hydroxide adjuvant (iep = 11.5) vaccine or a model lysozyme (iep = 11.4) and aluminum phosphate adjuvant (iep 5.0) vaccine to values ranging from 3 to 11 was studied. The model vaccine prepared at pH 7.4 served as the reference. Additional adsorption of ovalbumin occurred in the pH range 7.4 to 4.1. Elution of ovalbumin occurred when the pH was adjusted to values above 7.4 or below 4.1. Maximum elution occurred at pH 2.9 and 11.0. Elution of lysozyme from aluminum phosphate adjuvant occurred when the pH was adjusted to any value other than the reference pH of 7.4, More lysozyme was eluted at pH 2.9 than at pH 11.0. The adsorption/elution behavior could be explained by the effect of pH on : (1) the ionization state of the protein, 2) the solubility of the adjuvant, and (3) the electrostatic interaction between the protein and adjuvant.