A new two-dimensional double-immunodiffusion technique in gel plates, the two-cross technique, has been applied to the investigation of immunosystems in the pH range from pH 5.0 to pH 8.5. The results can be summarized as follows: (a) With increasing pH values of the medium, precipitating titers of all the proteins involved in the antigen-antibody systems examined increased. Consequently, the solubility of aggregates of the primary antigen-antibody complex decreased. (b) In the human albumin-rabbit antibody system the stoichiometry of the primary complex varies in the pH range from 5 to 7, the antigen-toantibody ratio at pH 5 being double the ratio at pH 7. (c) The antigen-to-antibody ratio of the system human IgG in serum-rabbit antibodies is constant from pH 5 up to pH 7, and then slightly changes in favor of the ratio. (d) Human albumin in serum steadily increased its molecular association with the rising pH, reaching a mean molecular association number N = 2.2 at pH 8.5, while the human IgG in serum was in a monomeric form from pH 5 up to pH 7.5, and then associated up to N = 1.6 at pH 8.5. (e) The mean association number of rabbit antibody preparations used was approximately N = 1.85 between pH values 5 and 7 and then increased up to approximately N = 3.2 at pH 8.5, depending on the batch and on the preparation procedure used.