In immunoassays such as homocysteine immunoassays, it is known that the measured values can be dispersed due to the influence of inhibiting substances present in a biological sample on the immunoreaction. The present invention provides among other things a method whereby the influence of the immunoreaction inhibiting substances is easily and efficiently suppressed or eliminated, and also provides a kit used for such a method. The influence of the immunoreaction inhibiting substances is suppressed or eliminated when the analyte (e.g., homocysteine) contained in the biological sample is reacted with the antibody in the presence of extrinsic polyanion.