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Analysis of a simple ligand/binding-protein association model predicts a decrease in the percentage of free hormone during ultrafiltration. Re-analysis of the original data validating ultrafiltration in free-ligand determinations (Sophianopoulos et al., Arch Biochem Biophys 1978;187:132-7) confirms this prediction. Binding data indicating a constant percentage of free hormone (Hammond et al., J Biol Chem 1980;255;5023-6) require a more complex ligand-protein interaction model. Application of these procedures to any new ligand system requires measurement of the percentage free, or of free-ligand concentration, as a function of filtration time or volume. The protocol becomes important if values for percentage free are to be accurate.

Two specific ligand-influenced monomer-dimer equilibrium systems are discussed. Each has a ligand-to-dimer subunit ratio of 0.5. Equilibrium characteristics of the system are described in terms of the effect of a bivalent ligand on both experimental and theoretical analysis. It is shown that Hill expressions need to be modified for multivalent ligand systems, and care is needed in equilibrium parameter determination. Some unique properties of these systems are retained, yet others are altered in the presence of a multivalent ligand. A suggestion as to the minimum amount of information needed to describe completely a ligand-influenced monomer-dimer system is given. The estrogen-receptor system is presented as an attractive biological model for both theoretical and experimental study of ligand-influenced polymerizing systems and their role in cellular control requirements.