We found a case that chimerism was suspected in ABO blood typing and performed flow cytometry analysis, microsatellite analysis, DNA typing for the HLA-A, B, C, DRB1 loci and the ABO blood group gene to establish a chimerism. This case is a 37-year-old male, and he has a dizygotic twins' younger brother. He was hospitalized with the fracture of his distal left radius. There is no history of blood transfusion. We tested the ABO blood group using a flow cytmetry. We extracted genomic DNA from peripheral blood, and then analysed microsatellites, the HLA-A, B, C, DRB, DQB1 loci and the ABO blood group locus. The flow cytmetry tests showed that the red cells with the B and the AB blood type was 90.04% and 9.96%, respectively. Three or four kinds of alleles were detected by the DNA typings and microsatellite analyses. From these findings, it was suggested that this case is a chimera in dizygote twins.