Conditions that affect the immunocytochemical [ICC] localization of tissue antigens are compared with known parameters found in radioimmunoassay procedures. Although the ICC localization itself suggests use of a monospecific antiserum, for all practical purposes, an antiserum even when raised against the most pure antigen contains several subpopulations of antibodies. These may differ from each other in concentration, avidity and specificity. Problems of concentration and avidity can be overcome by manipulating some of the conditions of the staining procedure. The only way to know what antigen is localized in a given ICC staining is to inhibit that staining by addition of the antigen to a working solution of the antibody. This procedure is called absorption or saturation.