In this paper the normal ranges of the expression of various differentiation antigens, referred to by their cluster of differentiation (CD) numbers, are described on the lymphoid, monocytic, and polymorphonuclear (PMN) blood populations in normal healthy individuals. The values expressed as antibody binding capacity per cell (ABC/cell) are related to the density of antigenic molecules expressed by these cells. These values have been quantitated by the quantitative indirect immunofluorescence (QlFI) test which renders the ABC/ cell values for the different antigens directly comparable and defines a "league table," i.e., an enumeration of antigen expression on the three main cell types studied. The values for occasional antigen that are expressed differently in adults and the elderly or men and women (CD5, CD8, and CD18) are also shown.Furthermore, the QlFl test is used in two-color immunofluorescence for defining the subset heterogeneity within the T lineage for the CD2 and CD7 antigens within the separately analyzed CD45RA and CD45RO subsets. These quantitative immune phenotype analyses, also referred to as quantimetry, show variations in ABC values if different monoclonal antibodies (MAbs) are used, although these differences are frequently minor. Therefore, using whole blood and well-characterized MAbs, we established values of antigen density in normal adults which can be regarded as control values for the various pathological conditions where CD antigen expression may be altered. o 1996 Witey-Liss, Inc.Key terms: leukocyte membrane antigen, antigen density, antibody binding capacity, flow cytometry, QlFl test, quantimetryRecently the stability and near linearity of the currently used flow cytometers have been well documented along the full range of fluorescence intensities (1). Collaborations have also shown that virtually identical flow cytometer settings can be obtained when multicenter studies adopt the same protocols and calibration procedures (2-4). Nevertheless, problems still arise from the fact that the preparation of antibodies and their conjugation with fluorochromes have not been fully standardized: antibodies differ depending on the fluorochromes used and are influenced by the conjugation methods, e.g., due to overconjugation or the use of spacer molecules during coupling, etc. These alterations can interfere with the binding of antibodies when compared to the features of the native antibody molecule.Despite these difficulties of standardization of immunofluorescence (IF), it is now accepted that the quantitation of molecules by monoclonal antibody (MAb) technology provides additional clinical information and is also useful for the precise characterization of lymphoid and hemopoietic cell populations. In certain malignant conditions precursor cell-associated antigens can be 0 1996 Wiley-Liss, Inc.overexpressed on malignant cells and can therefore be regarded as leukemia-associated features (5-7). In various pathological conditions, particularly in infectious diseases, the regulation of antigen ex...