A rapid new practical method for calculating both the antibody-antigen equilibrium constant and the antibody concentration from antibody dilution curve data alone is described. This method is faster than the inhibition curve method for evaluating a humoral immune response. It is particularly suitable for monitoring the immune response of an immunization program. The response is assessed as an immunization index, Abi*Ka. This index is more exact than the antibody titer obtained from dilution curves and independent of the specific activity of the labelled molecule and total activity used in the assay. The method was used to monitor the production of a monoclonal antibody to the sulphide peptide leucotriene including immunization, cloning and purification.