Farr Assay 
 
 The Farr assay (1, 2) is a precipitation-based immunoassay that is especially appropriate for analysis of certain antibody–antigen interactions that do not lead to an insoluble lattice of crosslinked antibody–antigen complexes. Failure to crosslink is frequently encountered with haptens, small antigens, and monoclonal antibodies. The Farr assay takes advantage of a common solubility property of immunoglobulins. Ammonium sulfate at 50% saturation causes antibodies and immune complexes to precipitate, whereas free antigens and haptens often remain soluble. Measurement of the antigen-binding capacity of an uncharacterized antibody preparation can be achieved by mixing a fixed amount of antigen with serial dilutions of antibody, allowing the binding reaction to reach equilibrium, precipitating free and complexed antibody with ammonium sulfate, and then measuring the antigen content of the precipitate. Antigen is precipitated quantitatively until a subsaturating dilution of antibody is reached, whereupon the antigen content of the precipitates progressively decreases. Observation of a constant amount of precipitate at saturating antibody concentrations differs from the precipitin reaction, in which excess antibody actually inhibits formation of a precipitate. The Farr assay is now obsolete, but is cited frequently in older literature.
References
1. R. S. Farr (1958) J. Infect. Dis. 103, 239–262. 
2. P. Minden and R. S. Farr (1967) "The ammonium sulphate method to measure antigen binding capacity", in Handbook of Experimental Immunology, D. M. Weir, ed., Blackwell, Oxford, pp. 493–526.