An expression is derived which allows the calculation of K-a without a value for total binding sites of antibody. The expression yields unambigous K-a values for homogeneous populations of antibody molecules. When heterogeneous populations are used, the value of calculated K-a rises as the fraction of sites bound falls. This expression should be useful in the determination of relative affinity with data obtained from Farr assays of the binding of anti-hapten antibody to hapten. Since values for total binding sites are unnecessary, this equation should be expecially applicable for the study of heterogeneous populations of relatively low affinity antibodies present at low concentrations.