It is important to identify the McLeod phenotype in order to differentiate the McLeod syndrome from other causes of acanthocytosis, e.g., chorea acanthocytosis. A proportion of males with the McLeod phenotype have X-linked chronic granulomatous disease. Because anti-Kx + -Km, which is needed for identification, is not readily available, detection of the McLeod phenotype relies on observed weakening of Kell antigens on the individual's red blood cells (RBCs). Identification of McLeod carrier females (obligate heterozygotes) is even more difficult because only a minor subpopulation of RBCs may express the weakened Kell phenotype. RBCs from 12 sets of mother/son or father/daughter pairs were tested by standard hemagglutination tube tests and by flow cytometry using both monoclonal and polyclonal Kell system antibodies. Monoclonal anti-K14 (G10) in tests with RBCs from McLeod males reacted +/- by hemagglutination (control cells 2+) and had a median fluorescence of 6-11 by flow cytometry (control cells 441). Monoclonal anti-k (F7) and human polyclonal anti-k (C30A-1) gave stronger reactions by hemagglutination with RBCs from McLeod males and were not appropriate to differentiate RBCs with the McLeod phenotype from RBCs with normal Kell antigen expression. Crisp mixed-field agglutination was obtained in tests with monoclonal anti-K14 versus RBCs from the 12 obligate carrier females. Flow cytometry using anti-K14 also clearly identified two RBC populations (median fluorescence of 6-11 and 229-382, respectively). Kell system antibodies may vary in their ability to detect a weakened expression of the corresponding antigen on RBCs with the McLeod phenotype by hemagglutination. Antibodies suitable for differentiating McLeod syndrome from other forms of acanthocytosis should be reagents that are preselected after extensive testing. In this study, flow cytometry clearly identified the two RBC populations in McLeod carrier females.