The degree of antigen adsorption by aluminum-containing adjuvants is considered an important characteristic of vaccines that is related to immunopotentiation by the adjuvant. This study examined immunopotentiation by aluminum phosphate adjuvant in three model vaccines in which the antigen was not adsorbed in the vaccine formulation nor when mixed in vitro with interstitial fluid. In the first model vaccine, aluminum phosphate adjuvant was pre-treated with 0.5 M KH2PO4 to minimize the adsorption of dephosphorylated alpha casein. The second model vaccine was composed of aluminum phosphate adjuvant and ovalbumin that was dephosphorylated by treatment with potato acid phosphatase. The third model vaccine consisted of aluminum phosphate adjuvant and lysozyme (LYS). In order to prevent adsorption of lysozyme, the aluminum phosphate adjuvant was pre-treated with fibrinogen, a protein present in interstitial fluid that binds strongly to aluminum phosphate adjuvant. Immunopotentiation was evaluated by measuring antibody production in mice. It was found that all three model vaccines induced antibody titers that were statistically higher than induced by a solution of antigen without adjuvant and similar to vaccines in which the antigens were adsorbed by aluminum phosphate adjuvant. Confocal microscopy experiments suggested that the antigens used in these experiments, even though not adsorbed to the aluminum phosphate adjuvant, were trapped in void spaces within the adjuvant aggregates, resulting in uptake of antigen by dendritic cells.