When a ligand X is multiply bound to energetically identical, noninteracting sites of a protein, cooperative binding of this ligand can be induced by the presence of a second ligand Y. This effect should appear whenever multiple interactions exist between the bound X and Y ligands, and vanish when the concentration of Y is made sufficiently large to ensure Y saturation at all concentrations of X. These predictions have been verified for the binding of 8-anilino-1-naphthalenesulfonate to serum albumin, when Y, the effector ion, is 3,5-dihydroxybenzoate. In the presence of 2mM dihydroxybenzoate, the Hill coefficient for anilinonaphthalenesulfonate binding rose steadily from 1 to 1.5 as the number of molecules of ligand bound increased from 1 to 3.3 per albumin molecule. The theory of interactions between isolated ligands, applied in the previous paper (D. A. Kolb and G. Weber (1975), Biochemistry, preceding paper in this issue), is extended to cases of multiple interactions, and applied here to show that the experimental results are tolerably well reproduced for a model in which four anilinonaphthalensulfonate molecules are homogeneously coupled to four molecules of dihydroxybenzoate by free energies of 3.0 and 3.5 thermal units.