Introduction: The importance of chimerism is that it is useful todemonstrate engraftment. It allows for early intervention which mightfacilitate treatment of emerging relapse in leukemia and it may help toprevent graft rejection.Aim of the work: The aim of this study was to evaluate mixed red cellspopulation and red blood cell chimerism after hematopoietic stem celltransplantation, to predict the final outcome of mixed chimerism whichmay help to decide interventions and prevent graft rejection.Material & Methods: This study was conducted on 22 bone marrowtransplanted patients admitted to BMT unit, Nasser institute, using simpleagglutination method and flow cytometric (FCM) technique.Results: In our study, 3 recipients had complete chimerism after 6 monthdetected by agglutination method. One recipient had complete chimerismafter 1 year detected by both simple agglutination method and FCM.2 recipients had mixed field agglutination after 30days detected byagglutination method and FCM.Conclusion: Evaluating erythrocyte repopulation by the agglutinationmethod is feasible, easy and cost effective.FCM analysis is a simple,sensitive test and an objective method, which could be used to detectchimerism stating 0.1- 0.3% sensitivity after allogeneic BMT. However,due to the difficulty found in retrieving the patients for follow up, wesuggest the use of alternative methods for detection of chimerism asVNTR and XY FISH probe.