Abstract To demonstrate the presence or absence of antibodies, results derived from a single serum dilution in an ELISA are sufficient. However, qualitative differences in antibodies are reflected by the shape of dose-response curves. A method based on approximating the absorbance value by a polynomial p(x) = a 1x+a 2x 2, where 1/x is the dilution factor, was used to characterize the dose-response curves in an ELISA for anti-ssDNA antibodies. The parameters used are E = a 1 and A = −a 1 2/a 2. It can be argued that E gives an estimate of the effective amount of antibodies in the serum and that A is essentially a function of the reaction constants between antibody and antigen.