The presence of polyreactive immunoglobulins in sera may substantially influence on the accuracy of antibody affinity determination. In order to obtain precise values of antibody affinity one should apply one of two following ways. First, one should block polyreactive immunoglobulins with high concentration of Twin 20 and high concentration of any antigen which does not interact with studying antibody. After this antibody affinity may be determined by traditional methods. Another way is the application of the method suggested by us earlier, which allow determining affinity of two antibodies in a mixture and the relation of their concentrations.