Chimerism is an important, yet uncommon cause of ABO phenotype/genotype discrepancies. The propositus was a 28 year-old pregnant women who had an ABO discrepancy, expressing an AweakB RBC phenotype and AB reverse type. Sequencing of ABO exons 6 and 7 revealed a B101/O01 genotype. Neither analysis of 9 short tandem repeats (STR) loci nor HLA typing on DNA extracted from white blood cells demonstrated evidence of chimerism. However, flow cytometric analysis using a PE-conjugated anti-A mouse monoclonal antibody detected a small population (3.8%) of red cells expressing normal A antigen. Based on this, we suggest that flow cytometric analysis is an effective method for the identification of small chimeric populations of RBCs in the immunohematology laboratory.